Explosions and heavy gunfire were reported in the Libyan capital Tripoli as rebel forces claimed they had launched an offensive against Muammar Gaddafi's forces in coordination with Nato.

Gun battles and mortar rounds were heard clearly at the hotel where foreign correspondents stay in the capital.

Explosions also sounded in the same area as Nato aircraft carried out heavy bombing runs after nightfall.

The fighting erupted just hours after the opposition captured the key city of Zawiya nearby.

Mustafa Abdel-Jalil, the head of the rebel leadership council, told the Arab satellite channel Al-Jazeera: "We planned this operation with Nato, our Arab associates and our rebel fighters in Tripoli with commanders in Benghazi."

Benghazi, hundreds of miles east of Tripoli, is the rebels' de facto capital.

Government spokesman Moussa Ibrahim appeared on Libyan television to deny that there was an uprising in Tripoli.

"Sure, there were some armed militants who escaped into some neighbourhoods and there were some scuffles, but we dealt with it within a half hour and it is now calm," he said.

The claims from both sides could not immediately be independently verified.

Rebels fighting to oust Muammar Gaddafi have scored a number of victories in the six-month civil war, only to see towns fall out of their hands. Now the momentum appears to have firmly swung in the opposition's favour after months of near-deadlock.

Col Fadlallah Haroun, a military commander in Benghazi, said the battles marked the beginning of Operation Mermaid - a nickname for Tripoli.

He also said the assault was coordinated with Nato. Col Haroun told reporters that weapons were assembled and sent by tugboats to Tripoli on Friday night.

"The fighters in Tripoli are rising up in two places at the moment - some are in the Tajoura neighbourhood and the other is near the Matiga (international) airport," he told Al-Jazeera.

Tajoura has been known since the beginning of the uprising in February as the Tripoli neighbourhood most strongly opposed to Gaddafi's regime.

Earlier yesterday, the government organised a trip for reporters to the airport to show them it was still in government hands.

The head of the rebel's leadership council said they chose to start the assault on Tripoli on the 20th day of the Muslim holy month of Ramadan, which fell on Saturday. The date marks the ancient Islamic Battle of Badr, when Muslims conquered the holy city of Mecca in AD 624.

A representative for Tripoli on the rebel leadership council told the AP that rebels were surrounding almost every neighbourhood in the capital, and there was especially heavy fighting in Fashloum, Tajoura and Souq al-Jomaa.

Those three neighbourhoods have been bubbling with discontent ever since the beginning of the Libya uprising. They paid the highest price in deaths when protesters took the streets in anti-Gaddafi protests, only to be met with live ammunition by government militiamen.

"We don't have exact numbers yet, but we are hearing that many fighters have fallen - very likely over 100," said Mohammed al-Harizi.

State television showed calm, aerial scenes of Tripoli in the daytime and shots of a very sparse Green Square in Tripoli, the epicentre of pro-Gaddafi rallies since the uprising began.

A couple hours after the rebels said they had attacked Tripoli, state television ran what appeared to be a live audio message by Gaddafi.

He did not appear on television but sounded like he was calling the message in on a poor phone line which crackled at times. He announced the time and date twice to prove that he was speaking live.

Gaddafi condemned the rebels as traitors and "vermin" who are tearing Libya apart and said they were being chased from city to city - a mirror image of reality.

"Libyans wanted to enjoy a peaceful Ramadan," he said. "Instead they have been made into refugees. What are we? Palestinians?"

Government spokesman Moussa Ibrahim appeared on Libyan television to deny there was an uprising in Tripoli. But he acknowledged that there was some kind of unusual activity.

"Sure there were some armed militants who escaped into some neighbourhoods and there were some scuffles, but we dealt with it within a half hour and it is now calm," he said.

The claims from both sides could not immediately be independently verified.