Amnesty International on Wednesday accused a powerful government-backed Libyan armed group of abuses against migrants and Tripoli residents.
In a statement, it accused the Stability Support Authority (SSA) of "unlawful killings, arbitrary detentions, interception and subsequent arbitrary detention of migrants and refugees, torture, forced labour, and other shocking human rights violations".
Amnesty said the group had been emboldened by a climate of "entrenched impunity".
The SSA, created under a decree by former prime minister Fayez al-Sarraj in January last year, is led by Abdel Ghani al-Kikli, one of the most powerful men in the North African country's capital.
Amnesty said al-Kikli, known as "Gheniwa", had been appointed despite a "well-documented history of crimes under international law and other serious human rights violations committed by militias under his command".
Libya plunged into violent lawlessness in 2011 with the NATO-backed revolt that toppled longtime dictator Moamer Kadhafi. Armed groups have vied for control of territory as a string of interim governments have come and gone.
Many such groups have been integrated into the state, partly in order to access a share of the country's vast oil wealth, and rights organisations have often accused them of abuses.
Since March the country has again found itself with two rival governments.
Amnesty said it had written to Libyan authorities last month to demand that al-Kikli and his former deputy Lotfi al-Harari be removed from "positions that would allow them to commit further violations, interfere in investigations or grant them immunity".
It said it had not received any response.
Harari now heads the Tripoli-based Internal Security Agency, another militia Amnesty accused of "crimes and human rights violations".
"Legitimizing abusive militia leaders and putting them on state payroll with no questions asked only empowers them to continue trampling on the rights of more people with complete impunity," said Amnesty's regional director Diana Eltahawy.
Last month Amnesty said armed groups affiliated with eastern Libya strongman Khalifa Haftar were detaining at least nine "peaceful protesters," reflecting how rights have been "brutally crushed" in areas under the control of Haftar-affiliated forces.
United Nations investigators said in March that serious rights violations including possible crimes against humanity were continuing with impunity across much of Libya, blocking the country's transition to peace and democracy.