Army units and militiamen loyal to Muammar Gaddafi have struck back against protesters, attacking a mosque where many had taken refuge and opening fire on others protecting a local airport.
A resident near the airport reported piles of bodies left behind and a "swamp of blood."
The assaults aimed to push back a rebellion that has moved closer to Gaddafi's bastion in the capital, Tripoli.
The revolt has already split off much of the eastern half of Libya.
In the latest blow to the Libyan leader, a cousin who is one of his closest aides, Ahmed Gadhaf al-Dam, announced that he has defected to Egypt in protest against the regime's bloody crackdown against the uprising, denouncing what he called "grave violations to human rights and human and international laws".
In the city of Zawiya, 30 miles west of Tripoli, an army unit attacked a mosque where protesters had been camping inside and in an area outside for several days, a witness said.
The soldiers opened fire with automatic weapons and hit the mosque's minaret with anti-aircraft missiles, he said. Some of the young men among the protesters had hunting rifles.
He said there were casualties, but could not give exact figures. He said a day earlier an envoy from Gaddafi had come to the city and warned protesters, "Either leave or you will see a massacre." Zawiya is a key city near an oil port and refineries.
"What is happening is horrible, those who attacked us are not the mercenaries; they are sons of our country," he said, sobbing. After the assault, thousands massed in the city's main Martyrs Square, shouting "leave, leave," in reference to Gaddafi, he said.
"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."
The other attack came at a small airport outside Misrata, Libya's third largest city, where rebels claimed control on Wednesday.
Militiamen today attacked a line of residents who were protecting the facility, opening fire with rocket-propelled grenades and mortars, said a resident who saw the assault
"They left piles of human remains and swamp of blood," he said. "The hospitals are packed with those killed and injured." But he could not provide exact figures.
After the attack ended before noon, another Misrata resident said the local radio, now in opposition hands, urged people to march on the airport in support of the protesters.
Both residents said the rebels continue to control the city, about 120 miles east of Tripoli. They and other witnesses around Libya spoke on condition of anonymity for fear of reprisals.
Gaddafi's crackdown has so far helped him maintain control of Tripoli, a city that holds about a third of Libya's six million population.
But the uprising by protesters, backed by army units that joined their ranks, has threatened to push the country toward civil war.
The leader's cousin, Gadhaf al-Dam, is one of the most high level defections to hit the regime so far, after many ambassadors around the world, the justice minister and the interior minister all sided with the protesters.
Gadhaf al-Dam belonged to Gaddafi's inner circle, officially his liaison with Egypt, but he also served as Gaddafi's envoy to other world leaders and frequently appeared by his side.
In a statement issued in Cairo today, Gadhaf al-Dam said he had left Libya for Egypt "in protest and to show disagreement" with the crackdown.