* Repeated reports of foreign fighters attacking protesters
* UN working group says their use looks increasingly likely
* Evidence points to fighters from west, central Africa
By Peter Apps, Political Risk Correspondent
LONDON, Feb 24 (Reuters) - Libyan leader Muammar Gaddafi, the loyalty of his armed forces proving decidedly unreliable, appears to have turned to mercenaries from elsewhere in Africa to support his bloody crackdown.
Witnesses and rights groups have told Reuters and other media repeatedly of foreigners brought in to fight, perhaps veterans of wars and insurgencies elsewhere in Africa -- often from countries with which Gaddafi has built strong links.
A lawyer in Benghazi said on Wednesday a security committee formed by civilians there had arrested 36 mercenaries from Chad, Niger and Sudan hired by Gaddafi's elite Praetorian Guard.
In Egypt, a 21-year-old Libyan student named Saddam said he had seen French-speaking fighters from west and north Africa open fire on protesters before he fled.
But Libya also has its own black African population, as well as thousands of African refugees hoping to cross to Europe. Identifying fighters' nationalities is hard.
Still, the head of the working group on mercenaries for the United Nations High Commissioner for Human Rights said evidence of the use of mercenaries was growing increasingly convincing.
"It's not 100 percent but it does seem likely," Jose Luis Gomez del Prado told Reuters. "It may be that the army are not willing to fire on their brothers and so it would make sense for Gaddafi to use them."
Citing reports from Libyan exile groups, the Paris-based International Federation for Human Rights said it believed Gaddafi was relying on up to 6,000 foreign fighters to cling to power, 3,000 of them based in the capital Tripoli.
The IFHR said foreign fighters came from many countries including Chad, Mali, Nigeria and Zimbabwe as well as Liberians who had fought for Charles Taylor, the former president now on trial for war crimes in neighbouring Sierra Leone.
Sudan's government said Darfur rebels long hosted by Gaddafi were also involved, a charge the rebels denied.
It may not be the first time Gaddafi has turned to outside help. During his fight against Islamist insurgents in the 1990s, there were persistent rumours he had hired Serbian mercenary pilots after Libyan officers refused to bomb civilians.
Some Libyan pilots again appear to have refused to obey his orders. Two pilots defected to Malta earlier in the week, saying they had refused to bomb protesters, while a Libyan newspaper reported a bomber crew ejected into the desert on Wednesday rather than bomb Benghazi.
The UN's Del Prado said he had heard occasional reports of eastern European mercenaries in the current crackdown, but most evidence pointed to fighters from elsewhere in Africa.
Chad, Democratic Republic of Congo, Liberia and others have all had bloody civil wars leaving whole generations traumatised and often armed. Gaddafi has built business and diplomatic links with many African countries in recent years.
Most accounts suggest any foreigners were brought into the country well in advance. Student Saddam said they had been hiding in army camps and emerged when troops began to mutiny.
"Many of the soldiers started leaving the camp, but the mercenaries fought with the army men trying to leave," he said.
Rights groups want an arms embargo in part to prevent Gaddafi bringing in any more fighters. They also want those guilty of any atrocities to be put on trial -- but experts say international justice may be the last thing on their minds.
"Mercenaries tend to worry about two things," said Adam Roberts, author of a book on the 2004 attempted coup in Equatorial Guinea involving mercenaries from southern Africa. "The first is whether they are going to get killed and the second is whether they are going to be paid." (Additional reporting by Leigh Thomas in Paris and Reuters reporters in Egypt and Libya; editing by Tim Pearce)