NATO warplanes have launched intensive air raids on Tripoli and its eastern suburbs, where several explosions were heard, an AFP reporter and witnesses said.
Four blasts shook the Libyan capital at around 2:30 am (0030 GMT) on Sunday, according to an AFP journalist who was unable to identify the targets.
Earlier, two powerful but distant blasts were felt in the centre of the capital at around 6:30 pm (1630 GMT) on Saturday, followed by several others a few minutes later.
Witnesses said the explosions came from Tajura, a suburb that has often been targeted by NATO since an international coalition began military operations against Libyan regime on March 31.
"For as long as Gaddafi continues to abuse his people, we will continue and intensify our efforts to stop him"
Britain's foreign secretary
Earlier on Saturday, William Hague, Britain's foreign secretary, and Andrew Mitchell, the development secretary, met with leaders of the Interim National Council (INC) in Benghazi, eastern Libya.
"We are here today for one principal reason - to show our support for the Libyan people and for the National Transitional Council, the legitimate representative of the Libyan people," Hague said in a statement.
Hague, accompanied by Mitchell, held talks with chief of the rebel National Transitional Council Mustafa al-Jalil.
He also toured Benghazi's landmark seafront as well as a medical centre treating war wounded.
"Gaddafi should leave immediately," Hague said.
He told reporters Britain would also support demining efforts in Misurata, the main rebel-held city in western Libya, and deliver "more equipment, uniforms, bullet-proof jackets" to rebel fighters.
"We have no combat troops in Libya," he added. But Britain, he said, would stand with the Libyan people "for as long as it takes".
"We could not and did not turn a blind eye when Gaddafi turned his forces against innocent civilians. For as long as Gaddafi continues to abuse his people, we will continue and intensify our efforts to stop him."
Hague's trip to the rebel capital came just hours after British Apache helicopters attacked forces loyal to Gaddafi.
The NATO offensive came as Libyan opposition fighters made a major advance towards the capital, Tripoli, after claiming victory in western Libyan towns against forces loyal to Gaddafi.
|Shashank Joshi, an associate fellow at RUSI in London, discusses the Apache's pros and cons
An opposition military leader said on Friday that local fighters won control of four towns in the Nafusa mountain range, where government forces had besieged and randomly shelled rebel-held areas for months.
Fighters who had fled then used their knowledge of the area to chip away at the government forces, Colonel Jumaa Ibrahim of the region's rebel military council told the Associated Press news agency via Skype.
"They know all the hills and valleys, so they were able to trick the brigades and destroy some of their vehicles," he said.
Opposition fighters have also pushed government troops from Shakshuk and Qasr al-Haj, two towns near a key road that runs along the mountain range's northern edge, Ibrahim, the rebel officer, said.
After a siege by pro-Gaddafi forces, Misurata, Libya's third largest city, is now in opposition hands.
Opposition fighters there have now pushed halfway to the town of Zlitan, on the way to Tripoli, after taking control of Zintan.
At one stage, their advance came to within 60km of Sirte, but the government troops held their line and repelled the attack.
On Friday, the Chinese foreign ministry said that China's ambassador to Qatar recently met the head of Libya's opposition council, the first known meeting between the two sides.
A Chinese foreign ministry statement said Zhang Zhiliang, Beijing's ambassador to Qatar, had met and "exchanged views on developments in Libya" with Mustafa Abdel Jalil, the chairman of the INC.
The ministry gave no details of the talks, but the meeting itself was an indication that Beijing wants to keep open lines of communication with the rebel forces.
China abstained in the UN Security Council vote authorising NATO military action in Libya.
Gaddafi's government has been slowly crumbling from within. A significant number of army officers and several cabinet ministers have defected, and most have expressed support for the opposition.
Rebels have turned down initiatives calling for ceasefires, insisting that Gaddafi and his sons must relinquish power and leave the country.