Mutassim Gaddafi, a son of Libya's former leader, has been arrested in his hometown of Sirte, the new National Transitional Council says.
"Mutassim Gaddafi was captured at Sirte and was transferred to Benghazi" in the country's east, NTC adviser Abdelkarim Bizama told AFP on Wednesday.
"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.
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Mutassim Gaddafi was born in 1975. He is a career soldier and doctor who held the title of national security adviser.
He had been considered a rival to his brother Seif al-Islam in the competition to succeed his father.
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 on Wednesday they were mopping up the last, increasingly weak and isolated pockets of resistance in Gaddafi's hometown, claiming its fall was imminent.