Heavy fighting raged Monday near the Tripoli compound of embattled Libyan leader Moamer Kadhafi, an AFP reporter said, a day after jubilant rebel forces surged into the symbolic heart of the capital.
Fighting was also heard from around 0400 GMT in the south of the capital, where there were exchanges of heavy weaponry and automatic rifle fire.
Rebel leaders had earlier warned that pockets of resistance remained despite most of Kadhafi's defenders vanishing during the rebels' lightning charge through Tripoli on Sunday.
The whereabouts of the Libyan strongman were unknown Monday but one of his sons, Seif al-Islam had been arrested while another, Mohamed Kadhafi was interviewed by Al-Jazeera television cowering in his house, afraid to leave.
Kadhafi broadcast three defiant audio messages on Sunday, vowing he would not surrender and urging the people of Tripoli to "purge the capital, even as rebel forces swept through the capital and took over the symbolic Green Square at the waterfront. But he has not been seen in public for weeks.
A diplomatic source said the strongman could still be in his Bab al-Aziziya compound in central Tripoli.
"He is still in Tripoli and could be in his residence at Bab al-Aziziya," said the source, who met the embattled strongman within the past two weeks.
The Bab al-Aziziya compound has been blasted regularly since the start of the international military intervention in Libya on March 19 and most of the buildings in the complex have been flattened.
But Kadhafi has many bunkers there that he could take cover in, the diplomatic source said.
US President Barack Obama said Kadhafi's 42-year autocratic regime was at a "tipping point" and that the "tyrant" must go, adding a call for the rebels to respect human rights and move to democracy.
Senior rebel figure Mahmud Jibril had earlier called on the insurgents to act responsibly as the battle to end four decades of dictatorship neared its end.
"The fight is not over yet," he said on rebel television Al-Ahrar. "God willing, in few hours our victory will be complete."
Thousands of residents poured onto the streets of Tripoli Sunday night to welcome the rebels, congregating at Green Square, which they renamed Martyrs Square.
Sky News showed jubilant crowds, with many people waving the red, black and green flag of anti-regime forces, dancing in joy and shouting Allahu Akbar (God is greatest). Some fired rifles into the air.
Similar scenes of jubilation were witnessed in Benghazi, the rebels' bastion in the east, where tens of thousands of delirious residents danced and proclaimed the end of the regime of the "tyrant" Kadhafi.
In his audio messages, the 69-year-old strongman vowed not to surrender and urged the people of Tripoli to "purge the capital."
Government spokesman Mussa Ibrahim told a press conference 1,300 people had been killed in the rebel assault on the capital, describing the fighting as a "real tragedy."
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But there was no independent confirmation of casualties, nor any immediate indication of how much resistance may have been put up against the rebels.
Ibrahim insisted that Libya's regime "is still strong and thousands of volunteers and soldiers are ready to fight" although the reality on the ground seemed to belie his boasts.
In The Hague, the International Criminal Court confirmed that Kadhafi's son, Seif al-Islam, for whom the ICC had issued arrest warrants for crimes against humanity, is in detention.
"I have received confidential information stating he has been arrested," Luis Moreno-Ocampo told AFP.
"We hope he can soon be in the Hague" to face justice, he said, adding that he planned to contact the "Libyan transitional government" later in the day.
Earlier, the chairman of Libya's rebel National Transitional Council (NTC), Mustafa Abdel Jalil told Al-Jazeera television from Benghazi Seif was "being kept in a secure place under close guard until he is handed over to the judiciary."
Abdel Jalil did not say when or where he had been captured.
In a brief telephonic interview with Al-Jazeera television broadcast Monday morning, another of Kadhafi's sons, Mohamed, said he was holed up in his house, frightened to leave.
During the broadcast the sound of intense firing could be heard, interrupting the interview. When it resumed, Kadhafi spoke with a tone of panic.
Al-Jazeera gave no indication of where the house is, or even whether it is in Tripoli.
Describing their assault, rebel leaders said an advance party of fighters had arrived by sea in the capital early Sunday and joined sleeper cells of rebels to launch the final drive, codenamed "Mermaid."
Another rebel force advanced from the west, moving in a convoy of around 100 vehicles as onlookers fired celebratory gunfire into the air, an AFP correspondent said.
By afternoon they had overrun the eastern suburb of Tajura and boasted that they would seize control of the capital during the night.
It was still not clear Monday morning how much of the capital the rebels had overrun and how long it would take them to mop up the districts where Kadhafi loyalists had dug in.
Guma al-Gamaty, the NTC spokesman in Britain, told the BBC that some loyalists were still heavily armed.
"I think the freedom fighters are dealing with some remnants of Kadhafi's forces -- some of the forces and some of the tanks and multi-rocket launchers and artillery are still positioned in certain places and are not willing to surrender yet," he said.
"They are being surrounded by the freedom fighters and they are given a chance to surrender peacefully rather than take them on in a bloody battle."
Obama issued a written statement calling on the rebels to respect human rights, show leadership, preserve the institutions of the Libyan state and move towards democracy.
"Tonight, the momentum against the Kadhafi regime has reached a tipping point. Tripoli is slipping from the grasp of a tyrant," Obama said as he took a vacation at the resort of Martha's Vineyard.
"The Kadhafi regime is showing signs of collapsing. The people of Libya are showing that the universal pursuit of dignity and freedom is far stronger than the iron fist of a dictator."