The Libyan government says Nato air raids have killed more than 700 civilians since bombing began in March.
Spokesman Moussa Ibrahim said more than 4,000 people had been wounded, but gave no evidence to confirm his figures.
Nato has denied killing large numbers of civilians, saying its air strikes are to protect Libyans from Colonel Gaddafi's forces.
Four powerful explosions were felt in the centre of Tripoli on Tuesday night, Libyan state media reported.
Planes were heard flying over the capital, but it was not possible to determine the targets of the raids.
'No exit strategy'
Speaking at a news conference in Tripoli, Mr Ibrahim accused Nato of killing and injuring hundreds of Libyan citizens.
"Since March 19, and up to May 26, there have been 718 martyrs among civilians and 4,067 wounded - 433 of them seriously," Mr Ibrahim said.
He said the figures did not include military casualties.
Foreign reporters in Tripoli have not been shown evidence of mass civilian casualties.
Asked why not, Mr Ibrahim said casualties had not been concentrated near the capital but scattered across the country.
He also denied that South African President Jacob Zuma, who met Col Gaddafi in Tripoli on Monday, had discussed an "exit strategy" with the Libyan leader.
"If Gaddafi goes, the security valve will disappear. His departure would be the worst case scenario for Libya," he told reporters.
A statement released by Mr Zuma's office after he returned to Pretoria said Mr Gaddafi would not leave Libya, despite growing international pressure.
"Col Gaddafi called for an end to the bombings to enable a Libyan dialogue," the statement read.
"He emphasised that he was not prepared to leave his country, despite the difficulties."
After initially backing Nato's involvement, Mr Zuma and the African Union have called for a halt to air strikes, arguing that Nato has overstepped its UN mandate to protect civilians.
Both Libyan rebels and Nato have refused to accept a ceasefire until Col Gaddafi agrees to step down.
On Tuesday, Italy's Foreign Minister Franco Frattini said that Gaddafi's regime was "finished", during a visit to the rebel capital Benghazi, in eastern Libya.
"He [Gaddafi] must leave office, he must leave the country," Mr Frattini told a joint news conference with Ali al-Essawi, the rebels' foreign affairs chief.
"His aides have left, he has no international support, the G8 leaders reject him, he must go."