Abdullah Senussi is the subject of arrest warrants issued by the Libyan and French governments. He's wanted by the International Criminal Court for his alleged role in the death of civilians during the Libyan civil war last year.
French authorities link him to a 1989 bombing of an commercial airplane over Niger that killed 170 people.
Senussi now faces a three-year prison term for allegedly entering Mauritania illegally using a fake passport from Mali, anonymous judicial sources told by The Guardian newspaper in London.
He's suspected by British authorities of playing a role in the bombing of Pan Am Flight 103 over Lockerbie, Scotland, in 1988. Abdelbaset Ali Mohmed al-Megrahi, the only man convicted of the bombing, died of cancer in Tripoli during the weekend.
Mauritanian officials were quoted by the British newspaper as saying Senussi was being held in a secret location. A formal trial isn't expected anytime soon, they added.