Rebel-held Zawiya, just 50km (30 miles) west of Tripoli, was the subject of a fierce government assault. Both sides later claimed to be in control.

Heavy casualties were reported there and in other key cities, including the eastern port of Ras Lanuf.

Dozens of people were also killed and hurt in apparently accidental blasts at an arms dump in rebel-held Benghazi.

Hospital sources in the city, Libya's second-largest, said they believed the two explosions were not triggered by an air strike.

Reports said at least 17 people had been killed in the blasts.

Earlier in the day, clashes briefly erupted after Friday prayers in the capital, Tripoli, but protesters dispersed after security forces fired tear gas and baton rounds.

The UN refugee agency UNHCR has expressed new concerns that people trying to flee into Tunisia may be finding their way blocked by armed pro-government forces, after a sudden drop in the numbers crossing the border.

At least 10,000 people a day were crossing the border earlier in the week, but the number suddenly fell to fewer than 2,000 on Thursday, the agency says.

Tens of thousands of people, most of them migrant workers, have streamed to the border since the unrest began, sparking a humanitarian crisis.

The European Union's humanitarian aid commissioner has demanded that Libya allow help into the country, citing increasing concerns over the situation of refugees in border areas.