© Tyler Hicks/The New York Times

Benghazi, Libya - Thousands of mercenary and other forces struck back at a tightening circle of rebellions around the capital, Tripoli, on Thursday, trying to fend off an uprising against the 40-year rule of Col. Muammar el-Qaddafi, who blamed the revolt on "hallucinogenic" drugs and Osama bin Laden.

The bloodiest fighting centered on Zawiya, a gateway city to the capital, just 30 miles west of Tripoli. Early Thursday, Colonel Qaddafi's forces arrived and unleashed an assault using automatic weapons and an anti-aircraft gun on a mosque occupied by rebels armed with hunting rifles, Libyans who had fled the country said.

An exiled Libyan who had been in contact with members of the opposition in Zawiya said the battle lasted four hours and had killed at least 100.

Fighting intensified in other cities near Tripoli as well - Misurata, 130 miles to the east, and Sabratha, about 50 miles west. Zuara, 75 miles west of the capital, had fallen to anti-government militias, other reports said.

To the east, at least half of the nation's 1,000-mile Mediterranean coast, up to the port of Ra's Lanuf, appeared to have fallen to opposition forces, a Guardian correspondent in the area reported.

"We are not afraid - we are watching," said a doctor by telephone from Sabratha. The city was under a state of siege, he said. Stores were closed and buildings belonging to the police and Colonel Qaddafi's revolutionary committees were in ruins after being burned by protesters. "What I am sure about," he said, "is that change is coming."

Colonel Qaddafi, speaking in an impassioned 30-minute phone call to a Libyan television station, appeared particularly incensed by the revolt in Zawiya, close as it was to the capital, and addressed the citizens there directly.

In a rambling discourse, he blamed the uprising on the leader of Al Qaeda, Osama bin Laden, saying he had drugged the people, giving them "hallucinogenic pills in their coffee with milk, like Nescafe."

"Those people who took your sons away from you and gave them drugs and said let them die are launching a campaign over cellphones against your sons, telling them not to obey their fathers and mothers, and they are destroying their country," he said.

The choice of peace or war, he said, belonged to the people of Zawiya - a town of 1,000 martyrs, as he called it - which had now become the focus of many of the thousands of forces he has called on to reinforce his stronghold in the capital.

Libyan state television flashed an urgent bulletin later Thursday, in English, saying: "We have seized voice recordings from some members of Al Qaeda who have joined in the city of Zawiya aiming to do sabotage actions."

Opponents of the government in Zawiya had been camped at the central mosque for days. On Wednesday, an envoy from Colonel Qaddafi came with a warning: "Either leave or you will see a massacre," one resident told The Associated Press. About 5 a.m. they fulfilled their threat. "Those who attacked us are not the mercenaries," he told The A.P. "They are sons of our country."

After the assault, he added, thousands rallied in the city's main Martyrs Square demanding that Colonel Qaddafi go. "People came to send a clear message: We are not afraid of death or your bullets," he said. "This regime will regret it. History will not forgive them."

Qaddafi loyalists also attacked in Misurata, where opponents of the government had claimed control Wednesday. Using rocket-propelled grenades and mortars, they struck at rebels guarding the airport, who seized an anti-aircraft gun used by the militias and turned it against them, The A.P. reported.

Residents of Tripoli, reached by telephone, said the uprising appeared to be headed toward a decisive stage, with Colonel Qaddafi fortifying his stronghold and protesters gearing up for their first organized demonstration after days of spontaneous rioting and bloody crackdowns.

"A message comes to every mobile phone about a general protest on Friday in Tripoli," one resident of Tripoli said.

In an apparent effort to demonstrate Colonel Qaddafi's control, his government announced on Thursday that it would allow teams of journalists to visit Tripoli, though without guaranteeing their safety. Reporters who entered the country illegally risked arrest and could be considered collaborators of Al Qaeda, the State Department warned.

One resident of the capital said the central Green Square - the scene of violent clashes earlier this week - had already been spruced up, with two banners hanging in English. "Al Jazeera, BBC, don't spread lies that reflect other's wishful thinking," one read. The other: "Family members talk but never fight between each other."

One of Colonel Qaddafi's sons, Seif al-Islam el-Qaddafi, said in an interview aired on Libyan state television that life was "quiet" in Tripoli; another son, Saadi, told The Financial Times that "50 or 60 percent of the people are working normally" in the capital. The protesters, he said, echoing his father, were under the influence of "very powerful" drugs like amphetamines and Ecstasy.

The overall death toll so far has been impossible to determine, but is clearly many hundreds. Even before the intensified battles on Thursday, Franco Frattini, the foreign minister of Italy - the former colonial power with longstanding ties - estimated that more than 1,000 had been killed. Tens of thousands of others are fleeing the country - to Tunisia, Egypt and Malta - including members of the government.

In the latest blow to the Libyan leader, a cousin who is one of his closest aides, Ahmed Gadhaf al-Dam, announced on Thursday that he had defected to Egypt in protest against the bloody crackdown, The A.P. reported.

The force that has attacked rebels on behalf of the government is one Colonel Qaddafi has built up quietly for years, distrustful of his own generals. It is made up of special brigades headed by his sons, segments of the military loyal to his native tribe and its allies, and legions of African mercenaries.

Many are believed to have fought elsewhere, in places like Sudan, but he has now called them back, relying on their willingness to carry out orders to kill Libyans that other police and military units, and even fighter pilots, have refused.

Colonel Qaddafi, who took power in a military coup, has always kept the Libyan military too weak and divided to do the same thing to him. About half its relatively small 50,000-member army is made up of poorly trained and unreliable conscripts, according to the Center for Strategic and International Studies.

Many of its battalions are organized along tribal lines, ensuring their loyalty to their own clan rather than to top military commanders - a pattern evident in the defection of portions of the army to help protesters take the eastern city of Benghazi.

Colonel Qaddafi's own clan dominates the air force and the upper level of army officers, and they are believed to have remained loyal to him, in part because his clan has the most to lose from his ouster.

Other clans, like the large Warfalla tribe, have complained that they have been shut out of the top ranks, said Paul Sullivan, a professor at Georgetown who has studied the Libyan military, which may help explain why they were among the first to turn on Colonel Qaddafi.

Untrusting of his officers, Colonel Qaddafi built up an elaborate paramilitary force - accompanied by special segments of the regular army that report primarily to his family. It is designed to check the army and in part to subdue his own population. At the top of that structure is his roughly 3,000-member revolutionary guard corps, which mainly guards him personally.

Then there are the militia units controlled by Colonel Qaddafi's seven sons. A cable from the United States Embassy in Libya released by WikiLeaks described his son Khamis's private battalion as the best equipped in the Libyan Army.

His brother Sa'ad has reportedly used his private battalion to help him secure business deals. And a third brother, Muatassim, is Colonel Qaddafi's national security adviser. In 2008 he asked for $2.8 billion to pay for a battalion of his own, to keep up with his brothers.

But perhaps the most significant force that Colonel Qaddafi has deployed against the current insurrection is one believed to consist of about 2,500 mercenaries from countries like Chad, Sudan and Niger that he calls his Islamic Pan African Brigade.

Colonel Qaddafi began recruiting for his force years ago as part of a scheme to bring the African nations around Libya into a common union, and the mercenaries he trained are believed to have returned to Sudan and other bloody conflicts around Africa. But from the accounts of many witnesses Colonel Qaddafi is believed to have recalled them - and perhaps others - to help suppress the uprising.