Abdullah al-Senussi, 63, was detained at Nouakchott airport, according to Mauritanian security officials
The former Libyan spy chief was Muammar Gaddafi’s brother-in-law and has been described as one of his most trusted aides.
Abdullah al-Senussi fled Libya when Gaddafi was ousted and killed last year after an uprising and months of fighting.
The International Criminal Court (ICC) issued a warrant for Abdullah al-Senussi’s arrest last year for crimes against humanity.
A French court convicted the former Libyan intelligence chief of involvement in a 1989 attack on a French plane that killed 170 people, and sentenced him to life in prison.
But Libyan authorities are also demanding his extradition.
Mauritania has not signed the ICC’s statute, and it is unclear what the country intends to do with Abdullah al-Senussi.
Mauritanian security officials said Abdullah al-Senussi was arrested during the night as he arrived on a regular flight from the Moroccan city of Casablanca on a false Malian passport.
Abdullah al-Senussi has been taken to the offices of the Mauritanian intelligence agency.
Libyan government spokesman Nasir al-Mani told state TV that Abdullah al-Senussi was travelling with a young man thought to be his son when he was arrested.
“The Libyan government is making contacts to demand that Abdullah al-Senussi be handed over,” said Nasir al-Mani.
Abdullah al-Senussi, nicknamed “the butcher”, was one of the last significant members of the regime still at large.
The former Libyan spy chief was indicted by the ICC along with Muammar Gaddafi and the leader’s son Saif al-Islam on 27 June 2011.
Saif al-Islam Gaddafi was captured in November in southern Libya and has been held by former rebels ever since.
The ICC wants him tried in The Hague but the Libyan authorities say he will receive a fair trial at home.
Libyan, Arab and Western sources describe Abdullah al-Senussi as a thuggish figure who would beat and abuse prisoners.
Abdullah al-Senussi is thought to have been responsible for purges of opponents within the regime in the 1980s and 90s, and for the deaths of 1,200 political prisoners at Tripoli’s Abu Salim prison in 1996.
He kept a low public profile during last year’s uprising, but reportedly played a key role in attempts to crush the revolt in the eastern city of Benghazi when it began last February.
There have been repeated reports of his death and capture which were later proved false.
Sources in the then opposition claimed Abdullah al-Senussi was killed in an attack by rebels in July in the Libyan capital Tripoli but later retracted the claim.
Officials in Niger said in October that he had fled through Niger into Mali, but a month later the new Libyan authorities said he had been arrested in the southern Libyan region of Sabha.
Further reports of Abdullah al-Senussi’s capture came in December but officials were unable to provide pictorial evidence.
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