The United Nations Libya mission (UNSMIL) called on Wednesday on Libyan authorities to free political activist Al-Moatassim Al-Areebi and to end arbitrary detentions.

Al-Areebi, 29, was abducted on Monday in the city of Misrata by unidentified armed men in civilian clothes along with his friend Mohamed Shtewi, the mission said in a statement.

UNSMIL said that Shtewi was released "after being beaten" but that the whereabouts of Al-Areebi "remain unknown".

UNSMIL urged Misrata city's security and law enforcement agencies to urgently investigate the abduction of Al-Areebi, disclose his whereabouts, and secure his safe and immediate release.

Misrata is a port city some 200 km (125 miles) east of the capital Tripoli. The Tripoli government is considered to be in charge of Misrata but has not commented on the case.

"Reports of arbitrary detentions, enforced disappearances, ill-treatment, torture, and deaths in custody committed with impunity continue to plague Libya," the mission said.

Libya has had little peace or stability since a 2011 NATO-backed uprising that ousted its leader Muammar Gaddafi.

It split in 2014 between eastern factions in Benghazi, the second-largest city, and western factions in Tripoli, with rival administrations governing in each region.

"The Mission has documented cases of at least 60 individuals currently detained across the country for their actual or perceived political affiliation," UNSMIL said in the statement.