Col Gaddafi's fugitive spy chief Abdullah al-Sanussi has been captured, Libya's interim government says.
He was seized by fighters in the south of the country, officials say.
Mr Sanussi, who has not yet been seen in custody, was one of the last senior figures from the Gaddafi regime still on the run.
Gaddafi's son Saif al-Islam was seized on Saturday. Both he and Mr Sanussi are wanted for alleged war crimes by the International Criminal Court (ICC).
Mr Sanussi, a brother-in-law of Col Gaddafi, is said to have been arrested at his sister's home in the southern town of Sabha on Sunday.
He was regarded as the late leader's right-hand man - and one of the regime's most-feared figures.
Mr Sanussi, 62, is being sought by the ICC in connection with the repression of protests against Gaddafi's rule earlier this year.
He has also been accused of human rights abuses, including his implication in the massacre in 1996 of more than 1,000 inmates at the Abu Salim prison in Tripoli.
Mr Sanussi was a close adviser to Saif al-Islam Gaddafi, according to leaked US embassy documents.
Saif al-Islam is due to be interrogated by prosecutors at a secret location in the western city of Zintan, where he was taken by militiamen who seized him in Libya's southern desert.
The commander of the Zintan militia told the BBC their prisoner was being well-treated at a private house, and had seen a doctor on Sunday about a wound to his hand.