Euphoric Libyan rebels last night swept into Tripoli. As 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."
Opposition fighters captured Gaddafi's son and one-time heir apparent, Saif al-Islam.
The prosecutor at the International Criminal Court in the Netherlands said he would contact the rebels to discuss Saif's handover for trial on charges of crimes against humanity.
Reporters with the rebels said the fighters rebels easily advanced 20 miles on Sunday from the west, took town after town - welcomed by residents - overwhelmed a major military base, then swept into the capital in a stunning turning of the tide in the six-month-old Libyan civil war.
The fighters and Tripoli residents who support them flooded Green Square, shooting in the air in celebration, clapping and waving the rebels' tri-colour flag, according to television footage of the scene.
Some set fire to the green flag of Gaddafi's regime and shot holes in a poster with his image.
"Now we don't call it the Green Square, but we call it the Martyrs Square," said Nour Eddin Shatouni, a 50-year-old engineer who was among the residents who flowed out of their homes to join the celebrations.
Green Square holds profound symbolic value. The regime has held pro-Gaddafi rallies there nearly every night since the revolt began in February, and the historic Red Fort that overlooks the square is a favourite scenic spot for the Libyan leader to deliver speeches to his loyalists.
Thousands of Libyans also celebrated in the streets of Benghazi, the rebels' de facto capital in the east. Firing guns into the air and shooting fireworks, they cheered and waved the rebel tricolour flags, dancing and singing in the city's main square.
Gaddafi's whereabouts are unknown, but he delivered a series of angry and defiant audio messages broadcast on state television, calling on his supporters to march in the streets of the capital and "purify it" from "the rats". He was not shown in the messages.
Government spokesman Moussa Ibrahim claimed the regime has "thousands and thousands of fighters" and vowed: "We will fight. We have whole cities on our sides. They are coming en masse to protect Tripoli to join the fight."
But it appeared that key parts of Gaddafi's military are peeling away.
The rebels' way into Tripoli was opened when the military unit in charge of protecting Gaddafi and the capital surrendered, ordering his troops to drop their weapons, the rebel information minister Mahmoud Shammam said.
In a sign of the coordination among rebels, as the main force moved into the city from the west, a second force of 200 opposition fighters from the city of Misrata further east landed by boat in the capital, bringing weapons and ammunition for Tripoli residents who join the rebellion, said Munir Ramzi of the rebels' military council in Misrata.
The rebels' speedy leap forward over just a few dramatic hours was a stunning reversal for Gaddafi, who earlier this month had seemed to have a firm grip on his stronghold in the western part of Libya, despite months of Nato airstrikes on his military.
Nato Secretary-General Anders Fogh Rasmussen said Gaddafi's regime was "clearly crumbling" and that the time to create a new democratic Libya has arrived.
US leaders are monitoring the progress of Libyan rebel forces as they move on Tripoli.
A State Department spokeswoman said: "Gaddafi's days are numbered. If Gaddafi cared about the welfare of the Libyan people, he would step down now."
President Barack Obama, who is on holiday in Martha's Vineyard, Massachusetts, told reporters: "We're going to wait until we have full confirmation of what has happened. I'll make a statement when we do."