As euphoric Libyan rebels swept into Tripoli, Britain called on Colonel Muammar Gaddafi to "go now" and save his people from further suffering.
With television pictures showing jubilant residents pouring onto the streets to welcome the rebel convoy, Downing Street said it was clear the "end is near" for the Libyan leader.
"It is clear from the scenes we are witnessing in Tripoli that the end is near for Gaddafi," the No 10 statement said."He has committed appalling crimes against the people of Libya and he must go now to avoid any further suffering for his own people."
Libya's ambassador to the United Nations Ibrahim Dabbashi, who switched to the rebel side, said: "This is not the beginning of the end, it is the end."
He told the BBC that Gaddafi could be "replaced" by rebel officials "within a few hours".
The rebels were reported to have advanced to within a mile and a half of Green Square where Gaddafi's supporters have been staging nightly shows of support for the leader.
Guma El-Gamaty, the UK spokesman for the National Transitional Council, told the BBC: "We are seeing the end now. Gaddafi has always maintained that as long as he holds on to Tripoli he still has some sort of power. Now that he looks like he's losing Tripoli, that's it."
In a statement, Nato said it was "ready to work with the Libyan people and with the Transitional National Council, which holds a great responsibility". The statement continued: "They must make sure that the transition is smooth and inclusive, that the country stays united, and that the future is founded on reconciliation and respect for human rights."
Meanwhile, the prosecutor of the International Criminal Court has said Saif al-Islam Gaddafi, a son of Col Gaddafi, has been detained by rebel forces. Saif Gaddafi was indicted along with his father earlier this year on charges of ordering, planning and participating in illegal attacks on civilians.
Prosecutor Luis Moreno-Ocampo confirmed Saif Gaddafi had been detained by "rebel special forces". He did not give the source of the information.