Pro-Gaddafi forces opened fire after hundreds of people in the Janzour district in western Tripoli started a protest march after Friday prayers, a resident, who asked not to be identified, told Reuters in an email.
He said protesters were also shouting anti-Gaddafi slogans in Fashloum in the city's east, and another resident said security forces had fired into the air there.
Al Jazeera said two people had been killed and several wounded in heavy shooting in several districts.
Tripoli and the surrounding area, where Gaddafi's forces had managed to stifle earlier protests, appear to be his last main stronghold as the revolt that has put the east under rebel control has also reportedly advanced through the west.
Zawiyah, an oil refining town on the main coastal highway 50 km (30 miles) west of Tripoli, has on successive nights fought off attempts by government forces to take control, said witnesses who fled across the Tunisian border at Ras Jdir.
"There are corpses everywhere ... It's a war in the true sense of the word," said Akila Jmaa, who crossed into Tunisia on Friday after travelling from the town.
Saeed Mustafa, who also drove through the town, said:
"There are army and police checkpoints around Zawiyah but there is no presence inside."
