Pro-Gaddafi forces have fired tear gas at protesters marching in the capital Tripoli.

The security forces fired at least five canisters of tear gas at the crowd of around 1,500 protesters in the district of Tajoura.

The crowd briefly scattered, but rejoined to continue their march, according to a reporter at the scene.

Fighters loyal to Muammar Gaddafi had set up checkpoints and searched cars, ahead of planned anti-government protests.

The opposition has called for protesters to march out of mosques after noon prayers.

Similar protests last Friday were met by gunfire.

More than 1,500 protesters marched out of the Murad Agha mosque after noon prayers chanting "the people want to bring the regime down" and waved the red, black and green flag of Libya's pre-Gaddafi monarchy.

The protesters transformed a nearby square, tearing down posters of the Libyan leader and replacing them with the flags. They spray-painted walls with graffiti reading, "Down with Gaddafi" and "Tajoura will dig your grave."

But soon after the march began, security forces fired tear gas at the crowd.

The protesters scattered, but rejoined to continue their march. Then security forces fired live ammunition, scattering the protesters again - though it was not immediately clear if they fired in the air or at people.

"I am not afraid," said one 29-year-old man among the protesters. He said in the protests a week ago one of his relatives was shot to death - not by militias, he said, but by a pro-Gaddafi infiltrator among 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.

Control of the capital is crucial to the Libyan leader, since it remains his strongest remaining bastion amid the uprising that began on February 15 and has broken the entire eastern half of Libya out of his control.

Even some cities in the west near Tripoli have fallen to the uprising, and the opposition has repelled repeated attacks by pro-Gaddafi forces trying to take back the territories.

A large force from a brigade led by one of Gaddafi's sons led a new attack today 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.

"Our men are fighting back the force, which is big," the resident said.

Throughout the night and into the early hours today, pro-Gaddafi forces also fired mortars and anti-aircraft guns at the outskirts of opposition-held Misrata, Libya's third largest city 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. Gaddafi'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, do not seem to have the capabilities to make a military move against territory still in regime hands.