Amnesty International says armed militia groups are threatening the security and stability of large parts of Libya.
According to Amnesty, groups of former rebels are committing human rights violations with impunity and Libya's new government has failed to prevent the abuses.
The organisation has issued a report to coincide with the first anniversary on Friday of the start of the revolt that toppled Colonel Muammar Gaddafi.
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.
Amnesty noted that African migrants and refugees - who were accused during the conflict of being "mercenaries" for Colonel Gaddafi - are among those being abused.
Last month, the UN said about 8000 pro-Gaddafi supporters were being held by militia groups.