An international aid agency has suspended its work in the Libyan city of Misrata because of what it says are persistent cases of torture.
Medecins Sans Frontieres says it has treated 115 patients with torture-related injuries at the centres, the BBC reports.
It said its medical staff were being "exploited" as some patients were being brought to them between interrogation sessions.
Director general Christopher Stokes says detainees being tortured are predominantly those suspected of having fought for former Libyan leader Muammar Gaddafi's forces.
Human rights group Amnesty International said several people had died after being tortured by militias in Libyan detention centres.
It claimed to have seen patients in Tripoli, Misrata and Gheryan with open wounds to their head, limbs and back.
More than 8500 detainees are being held by militia groups in about 60 centres, according to the United Nations.
UN Human Rights Commissioner Navi Pillay says the militias are working outside of government control.