Galvanized by anger over the killing of the popular American ambassador here last week, thousands of Libyans marched through this city on Friday, demanding the disarming of the militias that helped topple the dictatorship but have troubled the country with their refusal to disband.
In a show of mass frustration at the armed groups, protesters seized control of several militia headquarters on Friday night and handed them over to Libya’s national army in what appeared to be a coordinated sweep. They also stormed the headquarters of Ansar al-Sharia, a hard-line Islamist militia that has been linked to the attack on the United States Mission in Benghazi that killed the ambassador and three other Americans.
As members of Ansar al-Sharia fled their headquarters, protesters set at least one vehicle on fire, and there were unconfirmed reports that several had been wounded by the departing gunmen.
At the seized headquarters of another militia, protesters burned and pillaged a large number of weapons, and hundreds of looters could be seen walking away with automatic rifles and rocket-propelled grenade launchers.
The killing of the ambassador, J. Christopher Stevens, a well-liked figure in Benghazi because he had worked closely with the rebels who toppled Col. Muammar el-Qaddafi last year, appeared to be the catalyst for the protests on Friday, though hardly its only cause.
The militias, which started forming soon after the February 2011 uprising against Colonel Qaddafi began in this eastern Libyan city, emerged as a parallel and often menacing presence after his downfall in October 2011, seizing territory for themselves and asserting their authority over the fledgling government.
In western Libya, turf wars between militias resulted in regular street fights with heavy weapons. Months ago, members of Ansar al-Sharia brandishing weapons paraded through Benghazi and called for an Islamic state.
It was unclear whether the backlash against Ansar al-Sharia and the other militias on Friday represented an opportunity for the government to consolidate its power in the post-Qaddafi era or would lead to new violent confrontations.
But no weapons were left behind in most of the seizures, protesters and officials said, suggesting the militias had been anticipating such an event because of a buildup of resentment against them.
In a further sign that tensions had been stoked, some militia members accused Qaddafi loyalists of instigating the backlash. Mohamed Bazina, a spokesman for the Rafallah al-Sehati brigade, one of the militias whose headquarters were seized, said it had video evidence to prove it.
“This is a military coup against the true revolutionaries in the city of Benghazi,” he said. “Benghazi will not calm down.”
The attack on the American Mission in Benghazi that killed Ambassador Stevens, on the 11th anniversary of the Sept. 11 attacks, was an affront to many in Benghazi, which Mr. Stevens had made his base during the uprising. He became a familiar, cheerful presence at public events.
“We want justice for Chris,” read one sign among the estimated 30,000 Libyans, including families, who marched into Benghazi’s main square on Friday to protest in front of the chief encampment of Ansar al-Sharia.
Some held signs reading “The ambassador was Libya’s friend” and “Libya lost a friend.” Many protesters carried Libyan flags, and government police officers could be seen mingling with the marchers.
Members of Ansar al-Sharia held a counterdemonstration, and arguments erupted between the opposing sides, but no violence occurred, at least not initially. Protesters chanted: “You terrorists, you cowards. Go back to Afghanistan.”
Mr. Stevens and the others were killed in mayhem that was ostensibly provoked by anger over an anti-Muslim video that was made in the United States and has been roiling the Islamic world for nearly two weeks. But officials have said there are indications that part of the attack may have been coordinated and planned.
The organization and firepower used in the assault has also raised alarm in Washington about the possibility of links to Al Qaeda. But to Libyans, the assault underscored instability in a country where militias keep weapons at the ready.
The Obama administration has been careful about publicly assigning blame in the death of Mr. Stevens and the others until law enforcement officials, including the F.B.I., know more. But the administration has begun to call the killings a “terrorist attack.”
The change in language came as Congressional Republicans criticized the administration over what they called its failure to anticipate the problems in Libya. Some Republican lawmakers have moved to cut off aid to Libya as a result.
But one powerful Republican, Senator John McCain, counseled against such a move, citing the pro-American sentiments of some of the demonstrators who confronted Ansar al-Sharia on Friday.
“These brave people in Libya are friends of America,” he said. “They want our help and need our help. And we must continue to provide it to them, which is exactly what Chris Stevens would have wanted.”
The fatal attack on the United States diplomatic compound here was actually a two-pronged assault, according to survivors. After attackers overwhelmed security at the American Mission, the survivors congregated at a nearby villa, surrounded by friendly Libyan forces, and believed themselves to be safe and waiting evacuation to the airport. The ambassador’s whereabouts was not known.
About 2 a.m., the ambush began, with gunfire and mortar rounds striking where the survivors had taken shelter. Two of the guards, Tyrone S. Woods and Glen A. Doherty, were killed. Questions about the ambush remain, but those who were present said it was conducted with a great deal of accuracy.
While the others were at the villa, Mr. Stevens; a computer technician, Sean Smith; and a security officer moved to a designated “safe haven” for the night, but attackers doused the building with fuel and ignited it. It was not known whether they were aware it was inhabited. The guard, who has not been identified, escaped the building, but Mr. Smith and Mr. Stevens were asphyxiated.
Photo: Demonstrators after burning a car they said was full of ammunition as they stormed the headquarters of Ansar al-Sharia, the militia suspected in last week’s attack, in Benghazi, Libya.