Vice President Dick Cheney and White House spokesman Tony Snow had argued that Democrats wanted to raise what Snow called "a white flag in the war on terror," citing as evidence the defeat of a three-term Democratic senator who backed the Iraq war in his effort to win renomination.

But Bush aides on Thursday fought the notion that they had exploited their knowledge of the coming British raid to hit Democrats, saying the trigger had been the defeat of Democratic Senator Joseph Lieberman of Connecticut by an anti-war political novice.

"The comments were purely and simply a reaction" to Democratic voters who "removed a pro-defense Senator and sent the message that the party would not tolerate candidates with such views," said Snow.

The public relations offensive "was not done in anticipation. It was not said with the knowledge that this was coming," the spokesman said.

Snow said Bush first learned in detail about the plot on Friday, and received two detailed briefings on it on Saturday and Sunday, as well as had two conversations about it with British Prime Minister
Tony Blair.

But a senior White House official said that the British government had not launched its raid until well after Cheney held a highly unusual conference call with reporters to attack the Democrats as weak against terrorism.

An aide to Lieberman, who would have been one of the first Democrats to hear of the plot because he is the top Democrat on the Senate
Homeland Security Committee, said the lawmaker first heard of it late Wednesday.

On Wednesday, Cheney had suggested that Democrats believe "that somehow we can retreat behind our oceans and not be actively engaged in this conflict and be safe here at home, which clearly we know we won't, we can't, be," he said.

While some Democrats have opposed some steps in the war on terrorism, and more and more are calling for a withdrawal from Iraq, no major figures in the party have called for a wholesale retreat in the broader conflict.

But Bush's Republicans hoped the raid would yield political gains.

"I'd rather be talking about this than all of the other things that Congress hasn't done well," one Republican congressional aide told AFP on condition of anonymity because of possible reprisals.

"Weeks before September 11th, this is going to play big," said another White House official, who also spoke on condition of not being named, adding that some Democratic candidates won't "look as appealing" under the circumstances.

Mr. Cheney, who a spokesman said had been kept abreast of the investigation, suggested in his remarks Wednesday that the outcome of a Democratic primary in Connecticut could embolden "Al Qaeda types."

Republicans, facing tough midterm elections - and with a history, as Democrats noted, of spotlighting terrorist threats in election seasons - used the news from England to try to pound home their message that they were doing everything possible to keep the nation safe. Mr. Bush strode off Air Force One to television cameras to declare that the United States was safer from terrorist attacks than it was before Sept. 11, 2001, but remained in danger.

On Capitol Hill and in states where Republicans are facing tough re-election battles, party officials applauded the arrests by the British authorities as evidence of the administration's policies in fighting terrorism, and suggested that Americans might take a cue from the tougher antiterrorism statutes Britain has enacted. In line with their efforts to keep the election from being a referendum on Mr. Bush and instead make it a choice between two distinct approaches to national security and other issues, they used the arrests to portray Democrats as weak.

"Defeatocrats!" declared a statement issued by office of the House majority leader, John A. Boehner of Ohio, capturing the tone of Republican rhetoric as the news unfolded.

The Ohio Republican chairman, Bill Bennett, attacked Representative Sherrod Brown, the Democratic challenger to Senator Mike DeWine, for voting against some intelligence bills, "the very types of programs that helped the British thwart these vicious attacks."

In a sign of how the terrorism issue was roiling American politics, Mr. Lieberman echoed Mr. Cheney as he attacked his primary opponent, Ned Lamont, for his opposition to the war. He said Mr. Lamont's desire to withdraw troops from Iraq would result in victory for Islamic extremists.

At the very least, the arrests in Britain were viewed by both parties as something of an August surprise, the kind of event that can change the story line of a campaign. The critical question now is the extent to which the fall campaign will be fought over the war in Iraq, something Republicans would like to avoid at all costs, or the overall campaign on terror, the only major issue where Republicans have consistently held an advantage over Democrats.

But in a sign of how this campaign might be different, Democrats struck a tone notably different from the elections of 2004 and 2002, when for the most part their strategy was to try to turn the subject away from national security. This time, Democrats attacked Republicans as failing to improve airline security and, most of all, argued that the decision to invade Iraq had been a distraction that depleted United States resources and allowed the world to become more dangerous.

"The war in Iraq had nothing to do with the war against international terrorism, or very little to do with the war on terrorism," said James Webb, a former Reagan administration official running as a Democratic candidate for Senate in Virginia. "It has distracted our attention, it has pulled our forces in, and we are now in a situation where we have 135,000 on the ground, which affects our ability to do a lot of things that we would be able to do otherwise."

Mr. Webb said one of his main tasks in trying to unseat Senator George Allen, a Republican, was to try to disentangle Iraq from the war on terrorism. "They have tried to keep it together - they have to make it one in people's minds in order to cover the strategic error of Iraq," he said in an interview.

Senator Harry Reid of Nevada, the Democratic leader, said, "This latest plot demonstrates the need for the Bush administration and the Congress to change course in Iraq and ensure that we are taking all the steps necessary to protect Americans at home and across the world."

Republicans have successfully portrayed Democrats as weak on terrorism for two national elections in a row. Still, before this threat, Democrats showed signs that they were viewing problems in Iraq and the unpopularity of the war as ways of undermining Republicans on their signature issue. And some Republicans were concerned that the party might not be able to go to the well on national security a third time.

Republicans have become increasingly alarmed that the war might drag down incumbents. A senior Republican consultant with ties to the White House, who was granted anonymity so he could describe internal research for a Republican member of Congress, said he had recently conducted a focus group in a highly contested Congressional district in the Philadelphia suburbs.

He was shocked, he said, at the degree of hostility among Republicans toward the war, even accounting for the fact that Northeast Republicans are more moderate than their counterparts in the rest of the country.

The importance of the struggle by both parties on whether this was an election season debate about terrorism or about the war in Iraq was demonstrated in a New York Times/CBS News poll late last month. Forty-two percent of Americans said they thought the Republicans were more likely than Democrats to make the right decision about the war on terror. The same percentage said they thought Democrats were more likely to make the right decisions about the war in Iraq.

The White House had been aware for weeks that Britain was moving to shut down this plot. White House officials said that Mr. Cheney was kept abreast of the plot and the investigation, but that his comments on Wednesday, in a rare teleconference with news service reporters, were simply in reaction to what they said was an extraordinary political event, the defeat of a sitting senator.

A senior White House official on Air Force One, speaking on the condition of anonymity, dismissed the notion that there was anything wrong with these kinds of issues being mixed up in a political campaign.

"The issue is going to be discussed in the fall," this official said. "Are you saying if the Democrats talk about the war, we shouldn't? We will talk about the war, and we will talk about the consequences of the policies advocated by the Democrats."

WASHINGTON - Since the Sept. 11 attacks, aviation security has been tightened in many areas - but not all.

Before they were foiled, terrorists apparently planned to exploit some of the remaining weaknesses in aviation security by assembling improvised bombs right inside airline passenger cabins.

The 2001 hijackings led to bulletproof cockpit doors. Machines that could detect explosives in checked baggage were installed in commercial airports. Sharp objects that could be used as weapons were banned, and better-trained airport screeners were hired to look for them.

So it may have been predictable that terrorists would try something else: smuggle aboard liquids that could be turned into explosives, put them together with other bomb parts and then detonate them.

"We've armored the flight deck doors, so they won't take planes and use them as weapons," said Oregon Rep. Peter DeFazio, a leading Democrat on the Homeland Security Committee. "Bag screening is better. The easiest way to do it is on a person or carryon."

On Friday, security was expected to intensify in U.S. airports with airline passengers facing a double-screening process, the head of the airline industry's largest trade group said. The extra screening was designed to keep passengers from carrying aboard any liquids that might be fashioned into explosives.

Passengers and their carry-on luggage were to be examined not only at the main security checkpoint but also a second time at the boarding gate. The stepped-up screening began Thursday at 25 airports with flights bound for Britain, according to James May, president of the Air Transport Association.

Intelligence had indicated the terror plot unfolding in Britain involved using benign liquids that could be mixed inside an airplane cabin to make an explosive.

Noting that terrorists repeat their tactics, DeFazio pointed to the 1994-1995 attempt to blow up a dozen airliners simultaneously over the Pacific Ocean. The plot, code-named "Bojinka," involved liquid explosives smuggled onto planes in bottles of contact lens solution.

The response to the latest terrorist threat produced long lines at airports Thursday as security officials scrambled to put new measures in place and passengers adjusted to perplexing new restrictions.

With a dearth of security equipment that can detect explosives on passengers, U.S. security officials moved quickly to ban liquids from passenger cabins. By day's end British Airways had banned carry-on bags from all flights between the United States and Britain.

Earlier Thursday, carryons were barred from U.S.-bound flights to keep passengers from carrying liquids onto aircraft. Then the ban was extended to all flights between the United States and Britain.

British Airways carries by far the most passengers between the two countries. The airline runs 80 flights daily between Britain and 19 U.S. airports.

Pilots complained that they weren't told quickly enough about the threat.

"The pilots are the in-flight security coordinators," said Al Aitken, a retired pilot who is a member of the Passenger Cargo Security Group, which lobbies for better aviation security. "How can he be that without knowing the latest information on the imminent threat? How can he brief his crew on what to look for?"

Rafi Ron, former head of security at Israel's Ben Gurion Airport and now a security consultant in Washington, said terrorists always try to exploit new vulnerabilities.

"We've been investing 99.9 percent of our resources in technology with one single purpose: the detection of weapons," Ron said. "Terrorists will always be able to get around it."

"Pakistan played a very important role in uncovering and breaking this international terrorist network. There were some arrests in Pakistan which were coordinated with arrests in the UK," Aslam told AFP.

The arrests in Britain "followed active international cooperation between Pakistan, the United Kingdom and the United States," she said, adding that "cooperation on this was spread over a period of time."

A senior Pakistani security official said at least three people were arrested "who are linked to Al-Qaeda", the terror network of Osama bin Laden blamed for the September 11, 2001 attacks on the United States.

The official had no further details of when or where they were seized.

Britain said early Thursday that it had thwarted a plot to wreak "mass murder on an unimaginable scale" by the simultaneous mid-air bombings of what reports said were up to nine planes.

Britain's domestic Press Association news agency said the 24 people arrested were mainly of Pakistani origin, while Deputy Commissioner Paul Stephenson, from the Metropolitan Police, strongly hinted they were Islamists.

Another security source said intelligence agencies in Islamabad had provided key leads to Britain's secret service that enabled them to bust the plot.

"The actions that have been taken in London were made possible only with the close cooperation between Pakistani and British intelligence," the official told AFP on condition of anonymity.

"Pakistani intelligence cooperated and provided vital information that led to these actions."

Al-Qaeda has already been fingered as a possible culprit behind the plot, with US Homeland Security Secretary Michael Chertoff saying the operation bore the hallmarks of the group.

Pakistan has arrested dozens of senior Al-Qaeda militants, including the capture three years ago of Khalid Sheikh Mohammed, the alleged mastermind of the September 11 attacks which used airliners as flying bombs.

Mohammed was also behind a foiled plan to blow up 12 US-bound passenger jets over the Pacific Ocean in 1995, called "Operation Bojinka".

Pakistan came under the spotlight after the July 7, 2005 suicide attacks on the London transport system when it emerged that some of the British-born bombers had attended Islamic religious schools, or madrassas, here.

Military ruler President Pervez Musharraf launched a crackdown on extremist groups after the London blasts, which killed 56 people including the bombers, and ordered foreign madrassa students to leave.

The incident, in which no one was hurt, "happened shortly after takeoff from Queen Alia International Airport in Amman," Qatar Airways said in a statement.

"A passenger, an Eritrean national aged 22, tried to force his way into the cockpit of the Airbus A300-600 aircraft which was carrying 12 crew members and 267 passengers," the statement said.

"A Qatar Airways male cabin crew member overpowered him and with the assistance of three passengers restrained the man before the aircraft landed safely in Amman," where the man was handed over to Jordanian police, it said.

"Full security checks are being carried out by Qatar Airways, which is working closely with local police and airport authorities in Amman," the statement said, adding that the plane was expected to resume its journey to Doha later on Thursday.

Qatar-based Al-Jazeera television earlier reported the incident as an apparent hijack attempt.

Qatar Airways spokeswoman Salam al-Shawa told the news channel that the Eritrean passenger attempted to enter the cockpit carrying a box containing an unspecified substance.

He was prevented from entering by the crew and passengers. The captain declared a state of alert and flew the plane back to Amman, where the suspect was arrested by Jordanian authorities, she said.

Shawa subsequently told Al-Jazeera that the man had been carrying "a rope" and his motives were not yet known.

But Jordanian government spokesman Nasser Jawdeh told AFP that the Eritrean man had tried to enter the plane's toilet while the plane was taking off from Amman and a tussle ensued.

One passenger speaking to Al-Jazeera by telephone from Amman gave a similar account, saying the man, a resident of Doha, tried to enter the toilet during takeoff, triggering a squabble.

The incident occurred on the same day that Britain announced it had thwarted a plot to wreak "mass murder" by the simultaneous mid-air bombings of planes to the United States, causing jitters around the world.

Some 1.6 million people were expected to pass through Paris' rail stations during the coming four-day holiday weekend.

Clement also asked prison directors to report any person suspected of preaching Islamic fundamentalism in French jails.

Meanwhile striking workers at a Paris airport security firm criticised plans for all hand luggage to be 100-percent searched on all flights to Britain, the United States and Israel, saying they were unworkable.

"Until now we were supposed to search 50 percent of all luggage and that was already impossible," said Danielle Hanryon, a union leader at the ICTS security firm, one of the largest at Paris Charles de Gaulle airport.

However ICTS management denied there would be any problem putting the new rules in place, despite protests from around 100 workers who went on strike to ask for more staff.

"All safety measures in place have been upheld, including the most recent ones announced for certain destinations," said the company's director of operations Pascal Kerouanthon.

The terror alert level in French airports was left unchanged at red, the third highest on a four-colour scale, following the discovery of the alleged plot, though security was stepped up at British airline check-in counters.