Several people have died after being tortured by militias in Libyan detention centres, human rights group Amnesty International has said.
It claimed to have seen patients in Tripoli, Misrata and Gheryan with open wounds to their head, limbs and back.
Meanwhile, charity Medecins Sans Frontieres has suspended operations in Misrata after treating 115 patients with torture-related wounds.
The UN says it is concerned about the conditions in which patients are held.
"The torture is being carried out by officially recognised military and security entities as well as by a multitude of armed militias operating outside any legal framework," a spokesman for London-based Amnesty said.'Exploited'
"After all the promises to get detention centres under control, it is horrifying to find that there has been no progress to stop the use of torture," Donatella Rovera, from the charity, said.
Medecins Sans Frontieres said it was being "exploited" as some patients were being brought to them between interrogation sessions.
"Our role is to provide medical care to war casualties and sick detainees, not to repeatedly treat the same patients between torture sessions," said general director Christopher Stokes.
More than 8,500 detainees, most of them accused of being loyal to former Libyan leader Col Muammar Gaddafi, are being held by militia groups in about 60 centres, according to UN human rights chief Navi Pillay.
"The lack of oversight by the central authority creates an environment conducive to torture and ill treatment," she said.
"My staff have received alarming reports that this is happening in places of detention they have visited."
Fighting between armed groups is continuing in parts of Libya, three months after the official end of the revolution that ousted Col Gaddafi.
At least four people were killed in the town of Bani Walid on Monday but it is unclear whether the clashes were between rival militias or local militia and remnants of forces loyal to Col Gaddafi.
Libya's interim leader Mustafa Abdul Jalil has warned of the dangers of a civil war if militias are not disarmed.
As the country continues its transition from civil war to stable democracy, the government wants to replace the different armed groups with a national army and police.
Col Gaddafi was killed in his home town of Sirte in October 2011, some 42 years after seizing power in a bloodless coup.
In a detention facility in Misrata late last year, I met a man with deep scars all over his back. He had been whipped with electric cables shortly after his capture, on simple suspicion of supporting Muammar al-Gaddafi. In another prison, in Tripoli, a former pilot showed me the results of electric shocks on his arm.
Libyan officials have repeatedly promised to tackle the abuses which, in October, Amnesty International warned were "staining" the record of the new Libya.
But the stain is spreading. The experience of Medecins Sans Frontieres in Misrata - and of Amnesty International - suggests that torture is becoming more entrenched. The government can no longer claim that it is the work of rogue militias, and international concern is growing.
This is not the new page for human rights that many had hoped for in Libya.