While Libyan factions in Benghazi clash over control of army bases in the city, opposition forces further west have broken through government lines at Misrata, routing pro-Gaddafi units and seizing tanks, heavy artillery and rocket launchers.

Fierce fighting outside the besieged city that began with a government offensive on Saturday ended in what rebel commanders say was a rout, as opposition fighters advanced nine miles.

The collapse of government units was so complete that the rebels came upon a treasure trove of heavy artillery, tanks, armoured vehicles and small arms at an abandoned school complex outside the nearby town of Zlitan.

Meanwhile in Benghazi, Libyan factions clashed in the early hours with an armed gang they said was loyal to Muammar Gaddafi, the latest sign of growing lawlessness in the rebel-held east following the death of their military commander, Abdel Fatah Younis, apparently at the hands of rebel forces.

Rebel spokesman Mahmoud Shammam told reporters in the opposition capital that the clashes broke out when rebel forces attacked a militia that had helped around 300 Gaddafi loyalists break out of jail on Friday. Rebel forces surrounded the barracks in which the militia, which calls itself the Nida Brigade, had sheltered.

At least six rebels were killed in the clashes, he said, which involved rocket-propelled grenades and machine guns.

"At 8am, the barracks was brought under control. Thirty men surrendered and we took their weapons," Shammam said.

"We consider them members of the fifth column," he added, reflecting growing fears among the opposition that Gaddafi loyalists have infiltrated their ranks.

The 300 Gaddafi soldiers and loyalists who broke out of jail are apparently still at large.

In Misrata, rebels arriving at the abandoned arsenal found the keys still in the ignitions of trucks holding grad multiple-barrelled rocket launchers, enabling them to hook them to the heavy guns and drive them back across the lines.

"They just left these behind, they left grad trucks, they left some cars, they left weapons," said Abdullah Maiteed, of the rebel Arise Brigade, stationed on the main Tripoli-Misrata highway. "We nailed them."

Four of the huge 155mm guns, the largest battlefield artillery weapon in use among most armies, were seen by the Guardian driving back from the frontline late on Saturday night. Each was towed by multiple-barrelled grad rocket launcher, the cabs of the tan-coloured trucks still bearing the graffiti of the Gaddafi forces.

Rebels say the government front west of Misrata, where Gaddafi has deployed his most powerful force, the 32nd Brigade, commanded by his son Khamis, has effectively disintegrated.

"The resistance today was not that much. I don't know, maybe he doesn't have an army," said Mohammed Elfituri of the Faisal (Sword) Brigade. "We thought that it would be a hard work [but] we moved 15 kilometres."

If the rebels can find the ammunition – a big if, given the UN arms embargo – the arrival of these guns may herald a change in the balance of power around Misrata, allowing the rebels to match the heavy weapons of pro-Gaddafi forces.

Misrata's forces also entered the town of Zlitan, an objective for the past six weeks, to find it empty of government troops. But soldiers from the rebel Shaheed (martyrs) brigade said they were turned back by residents, who feared that a rebel advance would mean their homes targeted by government artillery.

Fourteen rebel fighters were killed and more than 60 wounded, with 40 government soldiers captured, among them seven pro-Gaddafi troops wounded and treated in Misrata. No figures for government casualties were available.

In the early hours of Sunday morning long range rockets struck Misrata city centre, killing three people.

While the mood of rebel troops in Misrata is confident, following news of gains by opposition forces in the Nafusa mountains to the west, commanders say they are cautious about predicting victory – Libya's rebel forces have yet to demonstrate the ability to launch sustained operations.

One opposition intelligence officer said Gaddafi may retain significant forces in the Wadi Ikam, a wide wooded valley further west on the road to Tripoli.