The UN has warned about widespread torture in Libyan detention centers run by revolutionaries who took arms against former dictator Muammar Gaddafi in 2011.

"The report indicates that torture is widespread and most frequent immediately after arrest and during the first days of interrogation to extract confessions and other information," Ravina Shamdasani, a spokeswoman for the Office of the High Commissioner for Human Rights (OHCHR), said on Tuesday.

"In some cases, members of the armed brigades freely admitted, and even tried to justify, the physical abuse of detainees," Shamdasani told reporters.

The authors of the report have documented their observations in 30 detention centers they visited over the past two years.

The OHCHR official said an estimated 8,000 remain in detention in centers run by armed brigades in connection with the Libyan revolution - mostly without due process.

According to the report, 27 cases of death occurred in such centers since late 2011 where torture was the likely cause, including 11 cases that took place in 2013.

Shamdasani called on Tripoli to take swift action to bring all facilities under full state control and renew “efforts to build the capacity of the criminal justice system.”

In February 2011, Libyans embarked on a revolution against Gaddafi’s four-decade rule and deposed him in August 2011. He was slain in his hometown of Sirte on October 20 of the same year.

Benghazi, Libya's second largest city and the birthplace of the 2011 revolution, remains largely in control of militias in the absence of unified Libyan security and military forces.

The former rebels refuse to lay down their arms, despite efforts by the central government to impose law and order.