The fighters have detained thousands of criminal suspects, former government officials and Qaddafi soldiers. Some are being held in prisons, others at makeshift, and sometimes secret, detention centers.

Some are being tortured. The ordeal of the 36-year-old detainee bore similarities to cases recorded by the group Human Rights Watch in six facilities administered by the anti-Qaddafi forces in Tripoli. In a report released Friday, the group said that detainees reported abuse including beatings and electric shocks. None of the 53 detainees interviewed, the group said, had been brought before a judge.

“What we’re seeing is a symptom of a fundamental problem,” said Tom Malinowski, the group’s Washington director. “Civilians have good plans but lack authority over the militia groups. The political vacuum at the top contributes to their weakness vis-à-vis the various armed groups, whether the concern is some abuse in prison facilities, or missing weapons or treatment of dark-skinned people.”