Bouzaid Dorda, the head of Muammar Gaddafi's external security organisation, has been arrested by anti-Gaddafi fighters, Reuters witnesses said on Sunday.
Dorda, Gaddafi's foreign intelligence service chief, will be handed over to Libya's interim governing council later on Sunday, an anti-Gaddafi fighter said.
A team of Reuters journalists visited a house in the capital's Zenata district where Dorda, a former prime minister, was held by members of a unit of anti-Gaddafi fighters who call themselves Brigades of the Martyr Abdelati Ghaddour.
Dorda was kept in the downstairs living room of a private house, which was guarded by about 20 fighters clad in battle fatigues and armed with assault rifles.
Dorda took on his job in May after his predecessor Moussa Koussa defected.
He has been subject to a travel ban under a United Nations sanctions resolution passed in February.
Dorda is one of several former government officials rounded up since Tripoli fell to anti-Gaddafi forces last month. Gaddafi's foreign minister, Abdelati Obeidi, was arrested on August 31 in a suburb west of Tripoli.