While the rebels claim to have taken several key towns, they are also meeting stiff resistance from forces loyal to the long-time leader.
Sustained blasts from rocket-propelled grenades, mortars and anti-aircraft guns rang out from the center of Zawiyah, some 50 kilometers west of Tripoli.
After hours of intense street battles, rebels re-took the city's main square.
They entered Zawiyah earlier this week. The latest clashes came as they tried to consolidate their control on the city and its strategic oil refinery.
On the eastern front, there were bloody street battles in the city of Zlitan. A rebel spokesman says the town has been captured, but at least 32 rebel fighters were killed, with 150 injured.
Meanwhile, rebels are also said to be in control of the town of Surman, which sits on the road between Tripoli and the Tunisian border in the West.
But Libyan government spokesman Mussa Ibrahim has denied Surman is in rebel hands.
Recent rebel advances have cut off the Libyan capital from its main supply routes, putting Gaddafi's four-decades rule under heavy pressure.