Thousands of residents poured onto the streets to welcome the rebels, congregating at the site which the renamed Martyrs Square near the water front in the centre of Tripoli.
Sky News late on Sunday showed scenes of jubilant crowds gathered there, many 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.
As many of the men flashed V-for-victory signs and shouted "tell Muammar (Gaddafi) and his sons that Libya has men," a Sky correspondent said people were lighting fires with posters of the Libyan strongman and the solid green flag of the regime that they had torn down.
Similar scenes of jubilation were witnessed in Benghazi, the rebels' bastion in the east, where delirious residents danced and proclaimed the end of the regime of the "tyrant" Gaddafi.
The strongman vowed not to surrender, even as NATO said his regime was crumbling and Britain predicted "the end was near" for the 69-year-old leader, who has kept a tight grip on power in his oil-rich North African nation for almost 42 years.
As the rebels boasted they would take full control of Tripoli during the night, Gaddafi issued his third message of the day, urging the people of Tripoli to "purge the capital."
Libyan government spokesman Mussa Ibrahim told a press conference that 1,300 people have been killed in the rebel assault on the capital, describing the fighting as a "real tragedy." There was no independent confirmation of the casualty toll, nor any immediate indication of how much resistance may have been put up against the rebels' entry into the capital.
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's prosecutor confirmed that Gaddafi'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.
Moreno-Ocampo said the ICC "is ready to help Libyans to deal with their difficult past" and ensure that "no crime remains unpunished".
Earlier, the chairman of Libya's rebel National Transitional Council (NTC) told Al-Jazeera television from Benghazi he had "information that Seif al-Islam has been captured".
"He is being kept in a secure place under close guard until he is handed over to the judiciary," Mustafa Abdel Jalil said, without say when or where Gaddafi''s son had been captured.