Several hundred opponents of Libyan leader Muammar Gadhafi spilled out of a mosque in the capital after prayers on Friday and started chanting: "Gadhafi is the enemy of God!"
Initially there was no sign of any security presence at the protest but pro-Gadhafi militias armed with Kalashnikov rifles had set up checkpoints around the neighborhood, reinforced by armored personnel carriers.
However, later in the day 14 sports utility vehicles carrying Libyan security forces sped through a checkpoint heading into an area of the capital where the anti-government protest had broken out.
Shooting broke out across the Tajoura district of Tripoli, and pro-Gadhafi forces in military fatigues and with green scarves on their heads arrived at the scene of the protest, firing tear gas at protesters.
A Reuters reporter said the Toyota vehicles had no registration plates, and the people inside were wearing military fatigues with green scarves on their heads. They were heading into the Tajoura district of eastern Tripoli.
"This is the end for Gadhafi. It's over. Forty years of crimes are over," said Faragha Salim, an engineer at the protest.
Tripoli is Gadhafi's principal stronghold after large swathes of the country rejected his rule, and the authorities have tried to portray it as a city going about its life as normal, but the protest punctured that image.
In rebel-held Zawiyah, about 50 km (30 miles) west of the Libyan capital, a witness said pro-Gadhafi forces attacked rebel positions to try to stop people attending Friday prayers in the town.
The authorities tried to prevent foreign media from reporting independently on the protests in Tripoli.
Security guards stood in the way when journalists including Reuters reporters tried to walk out of the gates of the media hotel to travel to neighborhoods in the capital where anti-Gadhafi protests are anticipated.
Officials later allowed them out of the hotel but only if they boarded buses with government drivers who were taking them to locations selected by the authorities.
Anti-Gadhafi protests continued throughout the country Friday. An attack was launched by a brigade led by one of Gadhafi's sons on Zawiya, the closest opposition-held city to Tripoli, a resident said.
The troops from the Khamis Brigade - named after the son - attacked Zawiya's western side, firing mortars and then engaging in battles of heavy machine guns and automatic weapons with armed residents and allied army units, said the resident.
Thuds of artillery fire and multiple explosions 20 km (12.4 miles) from the oil port of Ras Lanuf on the Libyan coast were also reported on Friday.
Throughout the night and into the early hours Friday, pro-Gadhafi forces also fired mortars and anti-aircraft guns at the outskirts of opposition-held Misrata, Libya's third largest city located just east of Tripoli, a doctor in the city said. He said it appeared to be an intimidation tactic, causing no casualties.
The crisis has turned into something of deadlock between the two sides. Gadhafi's forces have been unable to take back significant ground from the rebellion. At the same time, his opponents, made up of ragtag citizen militias backed by mutinous army units, don't seem to have the capability to make a military move against territory still in regime hands.
Gadhafi is facing international condemnation over the hundreds killed in a crackdown on the revolt.