Dorda, Qaddafi's foreign intelligence service chief, will be handed over to Libya's interim governing council later, an anti-Qaddafi 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-Qaddafi 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-Qaddafi forces last month. Qaddafi's foreign minister, Abdelati Obeidi, was arrested on Aug. 31 in a suburb west of Tripoli.
Meanwhile in a sign of divisions within the anti-Qaddafi ranks, at least 12 people were killed and 16 wounded when two groups of fighters turned on each other in the country's west.
The fighting, which has its roots in ancient rivalries and pitted combatants from the towns of Gharyan and Kikla on the one side and from Asabah on the other, broke out on Saturday, according to the chief of the Gharyan council and confirmed by the head of the military council of Asabah.
The towns are on the eastern edge of the Nafusa mountains and were important centers of resistance to Qaddafi's forces in months of fighting to oust the strongman.
"The fighting erupted when a brigade from Gharyan and Kikla came under fire at the town of Asabah," said Gharyan council head Wahid Barshan. He added that the brigade had been ambushed "after demanding the return of their heavy weapons" left behind during fighting against Qaddafi's forces.
The toll and the version of events were confirmed by the chairman of the Asabah military council, Saad Al-Shartaa.
Traditionally, many people in Asabah were Qaddafi supporters, and according to the two officials, 20 of his fighters were captured during Saturday's fighting.
Shartaa said efforts had been made "to contain the situation" and that Libya's new ruling National Transitional Council had intervened in a bid to calm passions.
He said the origin of the feuding lies in the fact that Asabah is the hometown of Qaddafi's one-time right-hand man Ahmed Ramadan.
