Britain has been on high alert since four suicide bombers killed 52 people in coordinated attacks on London's transport network on July 7 last year.

The government has tightened its anti-terrorism legislation since then and police can now detain suspects for up to 14 days before charging them.

Police are also on the alert for Britons they suspect of planning terrorist attacks abroad.

With a Muslim population of around 1.7 million, many angered by British foreign policy in the Middle East, Britain has been cited as a fertile recruiting ground for Islamist extremists.

In December 2001, Briton Richard Reid was arrested for trying to blow up a passenger plane with explosives hidden in his shoes.

He was sentenced to life in jail in January 2003 after a trial in which he described himself as an Islamic fundamentalist working in league with al Qaeda.

In 2003, two British Muslims attacked a bar in Tel Aviv, killing three Israelis. One of them, Asif Mohammed Hanif, blew himself up at the scene while the other, Omar Sharif, fled, and was later found drowned in the sea nearby.

U.S. authorities arrested nine Britons abroad in the wake of the attacks of September 11, 2001, and imprisoned them at the Guantanamo Bay naval base in Cuba.

None was ever charged with any offence and all have since been freed.

"Given the upcoming Memorial Day holiday, we advise American citizens to exercise caution and report any suspicious activities to authorities."

America's Memorial Day holiday is on Monday, May 29.

Japan's top government spokesman, Shinzo Abe, said he had no information that suggested there would be an attack in Japan.

"We do not have any concrete information that a terrorist attack will be carried out in Japan," he told reporters.

Since the September 11, 2001, attacks on the United States, security has been tight around the U.S. embassy and other facilities related to the United States in Japan.

From the May 22 edition of Fox News' Your World with Neil Cavuto:

CAVUTO: Tomorrow: Are Democratic leaders who criticize the war in Iraq actually aiding the terrorists? Why Nevada Senator John Ensign says, "You bet they are." And that's just the tip of the iceberg. You'll be surprised at what else he has to say. He's tomorrow on Your World.

Courts and public institutions in the United States, Britain and Europe called their governments to account for permitting torture and other abuses of terrorism suspects, she said, and the public has shown growing distaste for attempts to justify violations of rights.

"Powerful governments are playing a dangerous game with human rights. Those with power and influence - the United States, European Union members, China and Russia - have been either complicit or compromised by human rights violations."

The large powers' disregard for rights "paralyzed international institutions and squandered public resources in pursuit of narrow security interests," Khan said. As a result, "the world has paid a heavy price in terms of erosion of fundamental principles and the enormous damage done to the lives and livelihoods of ordinary people."

Both the United States and Britain have come in for sharp criticism by Amnesty since 2001 when new counter-terrorism laws opened the way for violations of rights.

Amnesty is one of many advocacy groups calling for closure of the U.S. detention centre at Guantanamo Bay, which has been attacked for overriding international conventions and individual human rights. Amnesty has asked for publication of the names and locations of terrorism suspects captured by American forces.

Calling Guantanamo "a powder keg waiting to explode," Khan said that "some 460 people of around 40 nationalities" remain there in spite of international protests, some of them going on hunger strikes or attempting suicide.

But, she said, while "there is evidence of widespread torture in U.S. detention centres, the United States (also) outsources torture to countries like Morocco, Jordan and Syria."

Washington has denied that it condones torture at Guantanamo or elsewhere, and insists that any abuses are isolated cases.

Khan also accused Britain of signing deals with Jordan, Libya and Lebanon to deport terror suspects, if it was confident they would not come to harm there. She urged Britain to examine the countries' human rights records and decide "whether those diplomatic assurances are worth the paper they are written on."

Citing the "duplicity and doublespeak" of the war against terror, Khan said European Union countries have not done enough to investigate claims that the CIA is using their airspace to transport terror suspects to countries where they might be tortured.

Western countries are also guilty of inaction on Sudan's wartorn Darfur region, she added, saying that "intermittent attention and feeble action by the United Nations and the African Union fell pathetically short of what was needed in Darfur."

Ending abuses in Darfur topped Amnesty's 2006 list of key human rights issues. It also called for negotiation of a new international arms trade treaty to halt the flow of weapons that promote human rights violations.

"The suspected perpetrators of these acts are not always made to account for them in court," said Geneviève Sevrin, president of Amnesty France.

"The racism of police officers and other state officials is aimed at persons of the Muslim faith or of ethnic minorities," Amnesty said.

The organisation also criticised a 2005 terrorism law which it said "authorises longer periods of secret detention and thus eliminates guarantees against the use of torture and ill-treatment and reinforces the impunity of the forces of law and order".

Amnesty also attacked the country's decision to place Albania and Georgia on the 'safe' list of countries whose nationals do not qualify for asylum.