Security forces have used teargas and live ammunition to disperse hundreds of anti-government protesters who marched in Tripoli after prayers.
Several hundred demonstrators gathered in Tajura, an area east of the capital, chanting: "Gaddafi is the enemy of God."
Protesters tore down posters of the Libyan leader and spraypainted walls with graffiti reading: "Down with Gaddafi" and "Tajura will dig your grave."
Scores of police cars descended on the area, forcing journalists from the scene, and at least one person was detained.
Soon after the march began, officers fired teargas at the crowd. The protesters scattered, but quickly regrouped before security forces fired live ammunition, scattering the protesters again. It was not immediately clear if the shots had been fired in the air or at the marchers.
"I am not afraid," one marcher told the Associated Press. The 29-year-old said one of his relatives had been shot dead in protests a week ago – not by militias, but by a pro-Gaddafi infiltrator at the demonstrations. "There are many spies among us. But we want to show the world that we are not afraid," he said, speaking on condition of anonymity because of fears of retaliation.
Earlier, security officials set up checkpoints in Tripoli backed by tanks and armoured personnel carriers. The checkpoints blocking many of the routes into Tajura, where demonstrations took place last week, were manned by well-trained forces who subjected vehicles to thorough searches.
Officials tried to stop foreign journalists leaving their hotel in Tripoli, claiming it was for their protection, but later allowed them out, although they were not permitted to go to Tajura.
The opposition called for protesters to march out of mosques after noon prayers in demonstrations against Gaddafi, who has vowed to fight to the "last man and bullet". In protests last Friday, pro-regime militiamen opened fire immediately on the marchers, killing and wounding a still unknown number. Since then, Gaddafi forces have seized suspected dissidents in night raids. Bodies of people who disappeared have been left in the street and militiamen have searched hospitals for the wounded to take away.
Opponents of the regime said they were expecting government forces to respond violently to this Friday's marches.
"We do not have any weapons. We will go to the mosque and then say Gaddafi should leave," Mohammed, a Tajura resident, told Reuters. "They [pro-Gaddafi militias] will attack."
As troops loyal to Gaddafi locked down the capital, a large force from a brigade led by one of his sons was reported to have attacked Zawiyah, the closest opposition-held city to Tripoli, about 30 miles away. A resident said the troops from the Khamis brigade, named after Gaddafi's son, attacked Zawiyah's western side, firing mortars and then using heavy machine guns and automatic weapons to fight armed residents and allied army units.
"Our men are fighting back the force, which is big," the resident told Associated Press. Zawiyah has beaten back several assaults in the past week in what is turning into a military stalemate. Gaddafi's forces have been unable to take back significant ground while his opponents, ragtag citizen militias backed by mutinous army units, lack the resources to take Tripoli.
Rebel forces are moving towards the major oil terminal of Ras Lanuf, 400 miles east of the capital. They are about 25 miles east of the port and expected to reach it by Friday evening or early Saturday morning.
"We're going to take it all, Ras Lanuf, Tripoli," one of the rebels, Magdi Mohammed, told Reuters.
Earlier, a Libyan warplane dropped bombs just beyond the walls of a military base, used to store huge amounts of ammunition, now held by rebels, in the eastern town of Ajdabiyah.
"We were sat here, heard the jet, then the explosion and the earth shook. They fell outside the walls," Hassan Faraj, who was guarding the munitions store at the Haniyeh base, told Reuters.
Meanwhile, the head of Libya's rebel national Libyan council has vowed to fight to victory or death.
"We are people who fight, we don't surrender," Mustafa Abdel Jalil, the former justice minister, told cheering crowds in al-Bayda in the rebel-held east of the country. "Victory or death. We will not stop till we liberate all this country … The time of hypocrisy is over."
The crowd chanted: "Libya is free and Gaddafi must go." Another chant said rebels would take their uprising to Tripoli. "The next round shall take place in the capital," they shouted.
Abdel Jalil said: "There could be members of the old regime here among us. Your enemy can still put his people among you. Don't listen to them and let them ruin our revolution."
He said the old regime was over. "You must keep all of Libya safe. Stop destroying buildings. They are ours now. It is being a bad Muslim to do something like this."
Abdel Jalil made the short speech in al-Bayda's central square, next to the burned out state security headquarters.
Barack Obama called on Thursday for Gaddafi to stand down on the grounds that he has "lost the legitimacy to lead" Libya after Tripoli's violent response to the protests. Obama issued his strongest condemnation of the Libyan leader as the international criminal court announced that Gaddafi and his sons are to be investigated for possible crimes against humanity.