Libyan rebel forces have seized control of much of Tripoli early Monday and arrested two of Colonel Muammar Gaddafi's sons, the International Criminal Court (ICC) confirmed.

Rebel fighters waved opposition flags to celebrate victory in Tripoli's Green Square on Aug. 21, 2011.

Rebel fighters waved opposition flags to celebrate victory in Tripoli's Green Square last night as heavy fighting continued, where Gaddafi's supporters had often rallied during the past weeks as a highly symbolic place.

The ICC prosecutor said one of Gaddafi's sons, Saif al-Islam, who has been indicted along with his father on crimes against humanity charges had been detained, the Guardian reported.

It also reported that Mustafa Abd el-Jalil, head of the rebel National Transitional Council, said his fighters who had detained Saif al-Islam had been given instructions to "treat him well".

Gaddafi's eldest son, Mohammed, and the presidential guard had surrendered but Gaddafi's Bab al-Aziziya compound was still under the control of the regime, it said.

Senior official Abdullah Almayhop from the National Transitional Council (NTC) said Sunday that opposition forces had controlled the entire capital except Gaddafi's Bab Al-Aziziyah stronghold and were clearing up the remnants of Gaddafi's troops, Xinhua reported.

Facing the offensive of the rebels, forces loyal to Gaddafi appeared to have crumbled quickly, and the guard unit responsible for Gaddafi's security had reportedly surrendered to the rebels.

At present, the whereabouts of Muammar Gaddafi remains a mystery. Gaddafi had vowed to fight to the last drop of blood and not to leave Libya.

Libyan government spokesman Moussa Ibrahim said Sunday that some 1,300 people had been killed in fighting in Tripoli on Sunday and that NATO should be held responsible for the bloodshed.

Moussa also asked to halt attacks and warned of "massacres to come in the city" by the rebels, The Independent reported.

Senior rebel leader Mahmoud Jibril, however, cautioned that "the fight is not over yet," while expressing the hope on rebel television that "in few hours our victory will be complete," Xinhua said.

The rapid collapse of Gaddafi's defense came though the embattled leader urged his followers twice on Sunday to pick up arms and fight against the rebels, calling it "the obligation of all Libyans" and "a matter of life or death."

The government was ready for immediate negotiations with the rebels, he added, while urging NATO to persuade the rebel forces to halt attacks at the capital.