LIBYAN militias are holding thousands of people in secret detention centres, while the interim government struggles to assert authority, the UN has heard.
The Security Council was told recent violence in Tripoli, Bani Walid and Benghazi highlighted the problem.
More than 8,000 pro-Gaddafi supporters are being held by militia groups, amid reports of torture, UN officials said.
The charity Medecins Sans Frontieres says it has suspended some operations because its work was being “exploited”.
The humanitarian medical organisation said it had stopped work in detention centres in the north-western city of Misrata because some patients were being brought in for care between interrogation sessions.
At least four people died in clashes in Bani Walid, a former Gaddafi stronghold, on Monday.
The UN’s Libya envoy, Ian Martin, told the Security Council in New York on Wednesday that those clashes between armed residents of Bani Walid and revolutionaries had been misreported as pro-Gaddafi forces retaking the city.
Nevertheless, he said, it highlighted the challenge of reconciling the former leader’s supporters and the rebels that had defeated them.
Militias were responsible for fatal clashes in Tripoli and fighting in other towns this month, he said.
“The former regime may have been toppled, but the harsh reality is that the Libyan people continue to have to live with its deep-rooted legacy,” said Mr Martin.
He described that legacy as “weak, at times absent, state institutions, coupled with the long absence of political parties and civil society organisations, which render the country’s transition more difficult”.
Mr Martin said some steps had been taken towards demobilising ex-combatants.
But the government was struggling to establish its legitimacy, he added, with weapons freely available and various armed brigades having unclear lines of command and control.
While authorities had so far successfully contained any outbreaks of violence, they could escalate and widen in scope, he warned.