Buoyed by gains on Saturday, rebel forces are pushing towards the Libyan capital, Tripoli, where there was heavy overnight gunfire and several explosions on Sunday morning.
A BBC correspondent with rebels who took Zawiya on Saturday says they have now taken Jaddayim, the first town en route to Tripoli, 40km (25 miles) east.
A government spokesman called for a ceasefire and accused Nato of working with the anti-Gaddafi forces.
Rebels are moving on two other fronts.
They took Zlitan, 160km (100 miles) east of Tripoli and are also advancing from the south, while Nato warships control access to the sea.
A rebel official said their supporters had started to rise up in Tripoli, but the violence appeared to peak late on Saturday night and there is still much support for Col Gaddafi in the city, correspondents say.
Pro-Gaddafi forces have been fighting back at the oil port of Brega, with the rebels admitting that they had fallen back from the eastern town's industrial zone under heavy bombardment.
'Massacres' Col Gaddafi's Information Minister Moussa Ibrahim accused Nato of going "mad".
"Nato in the last week or so... has been opening the roads ahead of the rebels who are too weak to do anything themselves," he told a news conference in Tripoli.
He also accused the rebel forces of carrying out "massacres" and killing people in the streets of those towns and villages they had seized in recent days.
He urged all parties to seek a peaceful way out of the crisis, warning that otherwise "many people will be killed and terrible crimes will be committed inside Libyan cities".
[h=2]"The zero hour has started"[/h]Abdel Hafiz Ghoga Transitional National Council
He also said that Tripoli was well protected by "thousands upon thousands of professional soldiers".
The BBC's Rupert Wingfield-Hayes in Jaddayim says hundreds of rebel fighters are in the town, both on foot and travelling in pick-up trucks. He says the town was taken after heavy fighting on Sunday morning.
Our correspondent says this is starting to feel like this may be the beginning of the march on Tripoli. This rebel advance seems to have more momentum than previous ones and has more support from Nato forces, he adds.
"We want to go to Tripoli today," one of the fighters, Bassam, told the AFP news agency, adding that Nato forces had been attacking the area's forest all night.
However, a fighter who returned from the front line to Zawiya said that pro-Gaddafi forces were pounding rebel positions with rockets and mortars.
In the capital itself, four loud explosions were heard on Sunday morning following hours of sustained gunfire in the city.
[h=2]At the scene[/h]Matthew Price BBC News, Tripoli
Colonel Gaddafi's Libya is under pressure like never before. Rebel forces are advancing towards the capital. They will expect to meet resistance before they arrive here. But the uprising may have started from within.
Overnight there were fierce clashes in several districts, with Col Gaddafi promising his people that the rebels had been "eliminated". Certainly the sound of the fighting indicated that troops had been ordered to fight them with all they have. The Libyan information minister called for an immediate ceasefire. He said a peaceful way out of the crisis needed to be negotiated, that the government had been saying this for months.
And he added that Nato would have blood on its hands if this did not happen. He said it was only with Nato support that the rebels were able to advance on Tripoli and that if they were allowed to enter, their priority would be blood and revenge.
There were overnight reports of protests and gunfire in areas to the north and east of Tripoli, including the Tajoura district, where there was trouble at the start of this uprising against Col Gaddafi, the BBC's Matthew Price reports from the capital.
The most intense period of fighting came at around 21:00 local time (19:00GMT) but the level of gunfire was much reduced by Sunday morning, our correspondent says.
The overnight fighting was almost certainly opponents of Col Gaddafi already in scattered parts of Tripoli rising up against pro-Gaddafi forces, rather than rebel forces advancing into the capital, our correspondent adds.
Abdel Hafiz Ghoga, vice-chairman of the rebel Transitional National Council (TNC), was quoted as saying by Reuters news agency: "The zero hour has started. The rebels in Tripoli have risen up."
He said the uprising had been co-ordinated with rebels forces elsewhere in the country.
But government officials say the attack was put down within half an hour.
In an audio broadcast shortly afterwards, Col Gaddafi congratulated his supporters for repelling the rebels.
"Those rats were attacked by the masses tonight and we eliminated them," he said.
His son, Saif al-Islam, ruled out any possibility of surrender.
Moussa Ibrahim: "Every drop of Libyan blood shed by the rebels is the responsibility of the western world"
"I see ourselves as victorious, I see our position is strong.
Source: BBCNEWS/RT