BENGHAZI, Libya — Militias loyal to Moammar Gadhafi fired in the air today to disperse marches by regime opponents defying a fierce clampdown to attempt their first major protest in the Libyan capital Tripoli in days.

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, chanting for Gadhafi’s ouster. They were confronted by a force of troops and militiamen near Green Square, said one witness. The militiamen fired in the air, sending some in the crowd running, he said.

“The situation is chaotic in parts of Tripoli now,” he said, adding that armed Gadhafi supporters were also speeding through some streets in vehicles. Other residents reported hearing gunfire in other districts of the capital.

The call for regime opponents to march from mosques after prayers was the first attempt to hold a major anti-Gadhafi rally in the capital — the Libyan leader’s biggest remaining stronghold — since bloody clashes Tuesday night. SMS messages were sent around urging, “Let us make this Friday the Friday of liberation,” residents 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 swept over nearly the entire eastern half of the country.

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.