Gun battles and rounds of mortar shelling were heard clearly at the hotel where foreign correspondents stay in the capital. Explosions were heard in the area as NATO aircraft carried out heavy bombing runs after nightfall.
Col. Fadlallah Haroun, a rebel military commander in their stronghold of Benghazi said this marks the beginning of Operation Mermaid -- a nickname for the capital city -- an assault on Tripoli coordinated with NATO.
Haroun told The Associated Press 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 neighborhood and the other is near the Matiga (international) airport," he told the Arabic satellite channel Al-Jazeera.
Earlier Saturday, the government organized a trip for reporters to the airport to show them it was still in government hands.
The head of the National Transitional Council said the date of August 20 was chosen to coordinate with the ancient Muslim Battle of Badr, when Muslims conquered the holy city of Mecca in A.D. 624.
"We planned this operation with NATO, our Arab associates and our rebel fighters in Tripoli and commanders in Benghazi," he said.
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 neighborhoods and there were some scuffles, but we dealt with it within a half hour and it is now calm," he said.
Rebels fighting to oust Gadhafi 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 favor after months of near-deadlock.