Armed militias are threatening the security and stability of Libya, Amnesty International has warned.
The human rights group says at least 12 detainees held by militias have died after being tortured since September.
The report is being released to coincide with the one-year anniversary on Friday of the revolt that toppled Libyan leader Muammar Gaddafi.
Last month, the UN said about 8,000 pro-Gaddafi supporters were being held by militia groups.
The interim government has said it is trying to reassert authority, but correspondents say it has largely failed to rein in the groups.
According to Amnesty, some groups of former rebels are committing human rights violations with impunity, unchecked by the interim government.
The report cites detainees who said they been suspended in contorted positions, beaten for hours with whips, cables, plastic hoses, metal chains and bars, and given electric shocks with live wires and taser-like electroshock weapons.
"Nobody is holding these militias responsible," Donatella Rovera, senior crisis response adviser at Amnesty International, told AP news agency.
In one detention centre, in Tripoli, investigators found severely tortured detainees who interrogators tried to conceal, Amnesty said. The report is based on research conducted in Libya in January and February.
The group noted that African migrants and refugees - who were accused during the conflict of being "mercenaries" for Muammar Gaddafi - were among those being abused.
Militias have also been responsible for fatal clashes in Tripoli and fighting in other towns in recent months.
On Tuesday, thousands of fighters from across western Libya held a parade in the capital, displaying heavy machine guns and rocket launchers, and firing rifles in the air.