Al Jazeera's Zeina Khodr reports from Green Square in central Tripoli

Euphoric Libyan rebels have moved into the centre of the capital, Tripoli, as Muammar Gaddafi's defenders melted away and thousands of jubilant civilians rushed out of their homes to cheer the long convoys of pickup trucks packed with fighters shooting in the air.

The rebels' surprising and speedy leap forward, after six months of largely deadlocked civil war, was packed into just a few dramatic hours. By nightfall on Sunday, they had advanced more than 32km to Tripoli.

Zeina Khodr, Al Jazeera's correspondent, said from the Green Square: "There's a party in the Libyan capital tonight. The people are in charge of the city. They've decided the square is now called Martyrs' Square, the original name. They're shouting 'We're free' and shooting at a poster of Gaddafi." 

Abdul Hakim Belhaj, the Tripoli battalion commander for the opposition National Transition Council said, "We want to thank God for this victory, we call on the residents to protect the city, we call on remaining parts of the regime to surrender and join the opposition."

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Green Square which has now been renamed Martyrs Square by the rebels had been the site of night rallies by Gaddafi supporters throughout the uprising.

Our correspondent said rebels met little resistance as they moved from the western outskirts into the capital.

"Hundreds are on the street, and most of them are armed. Most of these are fighters who came down from the mountains in western areas of Libya. They entered the capital a few hours ago and with the opposition inside the capital, have managed to liberate the city from the government's control," our correspondent said.

"People are worried about sleeper cells but cleaning up operations are under way to make sure there are no snipers in the buildings nearby. People are confident that the government has fallen and they are in control."

Pockets of fighters who are still loyal to Gaddafi still control parts of the city - including the areas around Gaddafi's Bab al-Azizia compound in the south of the city. The AFP news agency reported the sound of heavy gunfire in the area early on Monday morning.

The chairman of the rebel National Transitional Council Mustafa Mohammed Abdul Jalil said early on Monday: "I warn you, there are still pockets of resistance in and around Tripoli."

NATO's chief spokeswoman, Oana Lungescu, said there was no formal co-ordination between the military alliance, which has waged a months-long air campaign against Gaddafi, and the rebels on the ground.

"What we're seeing tonight is the regime crumbling," Lungescu told AFP news agency.

Saif al-Islam arrested

There was no word on the whereabouts of Gaddafi himself. Gaddafi has delivered a series of angry and defiant audio messages in recent days, vowing not to surrender. In the latest one, he acknowledged that opposition forces were moving into Tripoli and warned the city would be turned into another Baghdad.

"How come you allow Tripoli, the capital, to be under occupation once again?" he said. "The traitors are paving the way for the occupation forces to be deployed in Tripoli."

Earlier, the rebel leadership said that Gaddafi's son, Saif al-Islam, had been arrested in a tourist village in western Tripoli.

Luis Moreno-Ocampo, the prosecutor of the International Criminal Court, confirmed Saif al-Islam had been detained and said the ICC would speak to the rebel National Transitional Council about his transfer to The Hague where the court is based.

Saif al-Islam, his father and Libya's intelligence chief, Abdullah al-Senussi, were indicted earlier this year for allegedly ordering, planning and participating in illegal attacks on civilians in the early days of the violent crackdown on anti-regime protesters.

Meanwhile, Gaddafi's eldest son, Mohammed, surrendered to rebel forces and spoke to Al Jazeera shortly afterwards.

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In the interview, he took an apologetic tone and said it was a lack of wisdom that caused the revolution and crisis in Libya.

"I've never been a government or security official, however I can tell you the absence of wisdom and foresight is what brought us to here today. Our differences could have been solved easily," he said.

As he spoke though, his house was attacked and shot at and the interview ended with the sound of gunfire.

"I'm being attacked right now," he said. "This is gunfire inside my house, they're inside my house. There is no God but Allah - no God but Allah and Muhammad is his messenger."

However, the head of the National Transitional Council later told Al Jazeera that Mohammed was not hurt.

"Neither Mohamed Muammar Gaddafi nor any one of his family was harmed," Mustafa Abdel Jalil said. "He will remain in his house, and I guarantee his safety."

There were no confirmed reports about the whereabouts of other members of the Gaddafi family.

'Relinquish power'

US President Barak Obama said Gaddafi must "acknowledge the reality that he no longer controls Libya. He needs to relinquish power once and for all".

In a statement issued from Martha's Vineyard, where is on holiday, Obama said: "The future of Libya is now in the hands of the Libyan people." He promised to work in close co-ordination with the rebels and said the US would "continue to insist that the basic rights of the Libyan people are respected."

Rebels in the west have taken numerous towns in the past month.

There were unconfirmed reports that two South African air force planes were spotted at Tripoli airport.

Speculation was rife that the planes were there to ferry Gaddafi out of Libya, but Mahmoud Shamam, of the NTC told Al Jazeera that it was unlikely the planes were meant for that purpose as the airport was under rebel control. He added that he did not believe that Gaddafi is in Tripoli.

Mohammed Dangor, South Africa's ambassador to Libya, speaking from Johannesburg, told Al Jazeera he had no knowledge of any of his country's planes in Tripoli.

"I have no knowledge of any South African planes in Tripoli ... but Nato should know, since they control the airport and no plane can land without their permission," he said.

Jalil said the rebels would halt their offensive if Gaddafi announced his departure.

"We will give Colonel Gaddafi and his sons a safe passage out of the country," said Jalil.

Source: Al Jazeera and agencies