The United Nations Libya mission (UNSMIL) urged authorities in the eastern-ruled part of the country to investigate the death of an activist who had been in detention since last year.
Siraj Dughman died while in custody at Rajma military camp in Rajma town, about 27 km (17 miles) east of Benghazi, UNSMIL said on social media platform X.
The mission said it was "deeply saddened" and called on authorities to conduct a transparent and independent investigation into his death.
Benghazi's Internal Security Agency said in a video statement that Dughman died after he fell on his head while climbing sewage pipes in an attempt to escape from a bathroom window on Friday morning.
The agency said Dughman had been detained pending a case referred by the public prosecution concerning meetings that included discussions about the overthrow of political bodies and the military.