Mustafa Abdel Jalil told a news conference in the rebel stronghold of Benghazi that he hopes Gaddafi will be "captured alive, so that he will be given a fair trial."
Gaddafi may still be in Tripoli, as fierce fighting rages around his Tripoli residence, media reports said earlier in the day.
Government forces are refusing to surrender, even though the rebels have gained control of much of Tripoli. They also claimed to have captured three of Gaddafi's sons.
Meanwhile, Libyan leader Muammar Gaddafi's elder son, Mohammad, escaped, Al Jazeera said late Monday. He was captured by the rebels earlier Monday.
Mohammad, who was arrested in his house during an interview with Al Jazeera, fled with the help of one of the rebels, the report said.
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