Independent and often unaccountable armed groups still man checkpoints and run security in capital Tripoli [AFP]

Libya's former ambassador to France under Muammar Gaddafi has died less than 24 hours after being detained by an armed group in Tripoli, a rights organisation has said.

Omar Brebesh, who served in the Paris embassy as cultural attache from 2004 to 2008, and then as acting ambassador, was detained on January 19 and appears to have died from torture, Human Rights Watch (HRW) said on Friday.

A preliminary autopsy report obtained by the organisation, and photographs provided by Brebesh's family, showed that the cause of death was "multiple bodily injuries and fractured ribs" and that his body was marked by welts, cuts and the apparent removal of toenails.

On January 19, Brebesh turned himself in to the al-Shohada Ashura, or Ashura Martyrs, after being called for questioning by the militia's commander, Khalid al-Blehzi, Brebesh's son Ziad said.

The next day, the family was told that Brebesh's body could be found in a mortuary in the mountain town of Zintan, about 100km southwest of the capital.

The Ashura Martyrs come from Zintan, and a prosecutor there has opened an investigation into Brebesh's death, Human Rights Watch said.

HRW also read a report by the judicial police in Tripoli, which said that Brebesh had died from torture and that an unnamed suspect had confessed to killing him.

Brebesh's son recovered his father's body in Zintan.

"I saw his face. There was blood on his nose and mouth. But I didn’t see the rest of his body or his face from the other side," he told HRW.

"There was a bump on his forehead. After that, I kissed him and that was it."

Thousands detained

Brebesh's death was one of the highest profile killings of a former government official since rebels captured and killed Gaddafi, the ousted longtime leader, in late October.

Others have been arrested or fled the country: Gaddafi son Saadi is in Niger, while his more prominent scion, Saif, was captured and is being held by militias in Zintan who have refused to give him up to the interim National Transitional Council (NTC).

The report into Brebesh's apparent torture and killing comes days after Doctors Without Borders announced it was suspending its work in Misrata detention centres because militias there were torturing detainees while asking the doctors for medical assistance.

According to the International Committee of the Red Cross, Libya has about 8,500 detainees in roughly 60 facilities, few of which come under centralised NTC control.