PRESS ASSOCIATION -- Colonel Muammar Gaddafi's 40-year regime appears to be crumbling as euphoric Libyan rebels swept into Tripoli amid calls from Britain for the leader to "go now".

Jubilant fighters poured into the city's main square and celebrated with residents of the capital as Gaddafi's defenders melted away.

Opposition fighters captured the dictator's eldest son, Saif al-Islam Gaddafi, and the International Criminal Court said it would contact the rebels to discuss his handover for trial on charges of crimes against humanity. A spokesman for the rebels said another of Gaddafi's sons, Mohammed, is reported to have been in contact with the rebels and is asking for guarantees for his safety.

It came after Downing Street said it was clear the "end is near" for the dictator. "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."

Fighters advanced 20 miles from the west with little resistance, overwhelming a military base and then pouring into Tripoli.

Although there were reports that fighting is continuing in some districts, cheering, clapping and celebratory shooting broke out in Green Square, the symbolic heart of the regime, as ecstatic Libyans waved the rebels' tri-colour flag. Others set fire to the green flag of Gaddafi's regime.

Thousands also celebrated in Benghazi, and Libyans in Britain were said to be celebrating in London's Edgware Road.

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".

Nato Secretary-general Anders Fogh Rasmussen said Gaddafi's regime was "clearly crumbling" and that the time to create a new Libya had arrived.

In a statement, US president Barack Obama said Gaddafi and his regime must recognise that its rule in Libya has come to an end.