Up to 7,000 prisoners are held in dozens of makeshift detention centres in Libya more than two months after rebel forces toppled Muammar Gaddafi, amid serious allegations and some evidence of torture, the United Nations said on Friday.

Transitional authorities lack a clear system for screening and registering detainees, thus opening the door to ill-treatment, Mona Rishmawi, a senior official of the U.N. human rights office, told a briefing after a weeklong visit to Libya.

Prisoners include people arrested at checkpoints without identity papers, suspected mercenaries from several regions, pro-Gaddafi fighters captured on the battlefield or people whose names appeared on lists of people to be rounded up, she said.

There are thousands of people who are being held, we are talking about a large number. It could be up to 7,000, said Rishmawi, who is in charge of the office's rule of law branch.

The majority of these people have not gone through a (judicial) process. This of course is a recipe for abuse. There are indications of very serious issues going on.

In all, there are believed to be 67 makeshift detention centres across Libya, compared with a few central prisons during the 42-year-rule of Gaddafi, she added.

The International Committee of the Red Cross has visited at least 40 Libyan prison facilities, ICRC spokesman Steven Anderson told Reuters on Friday. Its confidential findings about conditions are shared only with detaining authorities.

Amnesty International issued a report on Wednesday saying Libya's new rulers were in danger of repeating human rights abuses commonplace under Gaddafi. The NTC said it would look into the report.

EVIDENCE OF TORTURE

Hanny Megally, head of the U.N. human rights office's Asia, Pacific, Middle East and North Africa branch, led the mission to Libya. They met ministers of the National Transitional Council, as well as activists and lawyers, and visited some prisons but did not speak with inmates.

...We actually believe that the situation in the prisons is that there is ill-treatment, there are allegations and evidence of torture. Yes, I can say that, Rishmawi said.

Libyan government forces pushed tanks deep into the city of Sirte on Friday to try to smash the last pocket of resistance by Gaddafi loyalists in his home town.

Rishmawi said that Libya lacked a coherent, centralised judicial system protecting prisoner rights, although judges in Benghazi and Misrata were starting to look at the cases.

It's a key challenge. And it will be even more challenging when the two big cities, Sirte and Bani Walid, fall. This will be a real issue for them, she said.

It would be wrong to assume all people still in Bani Walid are pro-Gaddafi fighters, they may include civilians who lack money or vehicles to leave or are too ill to travel, she added.

Right now we can say that the system that is currently in place is not adequate. There is a lot of room for abuse although I think that the policy at the higher level is that 'we will not tolerate torture, we will investigate'.

Gaddafi's whereabouts are unknown, but most Libyans are keen to see him and his family face trial, Rishmawi said.

They really want to see that day, she said.

Luis Moreno-Ocampo, prosecutor of the International Criminal Court (ICC), indicted Gaddafi for war crimes last May.

An international commission of inquiry on Libya, led by U.S.-based expert Cherif Bassiouni, has previously accused his forces of murder, torture and abduction.

It will make its third visit later this month to further investigate abuses including alleged mass rapes, Rishmawi said.