At least five people were killed when security forces opened fire on anti-government protesters in the Janzour district in the west of Tripoli on Friday, a witness said.
This was the first major protest in the Libyan capital in days.
The witness, who did not want to be identified, also said opponents of Libyan leader Muammar Gaddafi were shouting anti-Gadhafi slogans in the Fashloum district, in the east of
Tripoli. The reports could not immediately be verified.
Militias loyal to Gadhafi initially fired in the air Friday to disperse marches by regime opponents defying the brutal clampdown that has left scores of Libyans dead.
Across rebellious cities in the east, thousands held rallies in support of the Tripoli protesters.
Protesters streamed out of a mosque in central Tripoli after prayers Friday, chanting for Gadhafi's ouster, and they were confronted by a force of troops and militiamen near Green Square, said one witness.
"The situation is chaotic in parts of Tripoli now," another witness said, adding that armed Gadhafi supporters were also speeding through some streets in vehicles.
The call for regime opponents march from mosques after prayers was the first attempt to hold a major anti-Gadhafi rally in the capital - the Libyan leaders biggest remaining stronghold - since bloody clashes Tuesday night.
Hundreds of protesters at the Slatnah Mosque in the Shargia district of Janzour were reportedly chanting anti-Gaddafi slogans, such as "With our souls, with our blood we protect Benghazi!"
Another Tripoli witness told a Reuters correspondent in Benghazi that snipers in the Libyan capital were killing people.
Ali, a businessman who declined to give his full name, told Reuters by phone that he was standing near a mosque on a road leading to the central Green Square. There was a crowd gathered in front of the mosque.
"They just started shooting people. People are being killed by snipers but I don't know how many are dead," he said.
Mohammad, 42, in Benghazi, said the Tripoli residents believed that if the demonstrators managed to mass in Green Square "that would spell the end for the regime".
SMS messages were sent around urging, 'Let us make this Friday the Friday of
liberation,' witnesss said.
Starting Friday morning, Gadhafi militiamen set up heavy security around many mosques in the city, intimidating opposition worshippers.
Armed young men with green armbands to show their support of Gadhafi set up checkpoints on many streets, stopping cars and searching them. Tanks and checkpoints lined the
road to Tripoli's airport, witnesses said.
Tripoli, home to nearly a third of Libya's 6 million people, is the center of the territory that remains under Gadhafi's control after the uprising that began Feb. 15 and swept over nearly the entire eastern half of the country, breaking cities there out of his regime's hold.
Even in the pocket of northwestern Libya around Tripoli, several cities have also fallen into the hands of the rebellion.
Militiamen and Gadhafi forces on Thursday were repelled in trying to take back territory in the cities of Zawiya and Misrata in fighting that killed at least 30 people.
Saif al-Islam Gadhafi, son of the Libyan leader, said Friday that his father will never resort to destroying Libya's oil wealth in its fight to put down an insurrection in an interview with Turkish news channel CNN-Turk.
He also said the Gadhafi family had no intention of fleeing Libya, and the government was in control of the west, south and center of the country.
He emphatically added that the Gadhafi family will live and die in Libya.
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