The reported arrest of Mutassim Gaddafi, a son of Libya's ousted leader, sparked celebrations on the streets of Libya, but the capture is yet to be officially confirmed.
Mutassim Gaddafi was arrested in his hometown of Sirte on Wednesday, the new National Transitional Council said.
"Mutassim Gaddafi was captured at Sirte and was transferred to Benghazi" in the country's east, NTC advisor Abdelkarim Bizama told AFP.
"We did not announce the capture earlier to avoid that (his family or aides) try to free him," he added.
Gaddafi opponents fired shots in the air in the capital Tripoli and Misrata after news of Mutassim's arrest broke.
Mutassim Gaddafi was born in 1975. A career soldier and doctor, he was appointed head of national security by his father in 2007.
He had been considered a rival to his brother Seif al-Islam in the competition to succeed the Libyan leader.
NTC officials had said repeatedly they suspected he had fled to Sirte, one of two remaining Gaddafi strongholds.
Seif al-Islam is believed to be hiding in Bani Walid, possibly with his father.
Forces of Libya's new regime said earlier Wednesday they were mopping up the last, increasingly weak and isolated pockets of resistance in Gaddafi's hometown, claiming its fall was imminent.
While diminishing in scope, fighting continued to shake the city throughout the day.
An AFP correspondent reported a fierce firefight around a school where Gaddafi loyalists were putting up strong resistance. He saw at least six bodies and said dozens of fighters had been wounded.
The plight of stranded civilians has raised the concerns of Human Rights Watch, which called on both sides to minimise harm to them and ensure that prisoners are treated humanely.
The NTC forces began their siege of Sirte on September 15 before launching on Friday what they termed a "final assault" that has seen at least 85 of their number killed and hundreds wounded, according to medics.
