Armed groups in Crimea have abducted and tortured two pro-Euromaidan activists, Human Rights Watch (HRW) said.
The activists Andriy Schekun and Anatoly Kovalsky were held for 11 days in secret detentions, along with other detainees.
Kovalsky and Schekun told HRW that on 9 March a group of men detained them at the Simferopol train station.
"These people did not charge us with any crime, did not ask us to sign any paperwork," Kovalsky said. "And yet they interrogated and tormented us."
Kovalsky and Schekun said their captors claimed they were members of Crimea's "self-defence" units.
Schekun also alleged to have been subjected to electric shock torture by a group of men in civilian clothing, whom he believed to be Russian security service agents.
The activists were handed over to Ukrainian military officers at the Chongar checkpoint, on Crimea's northern administrative border, on 20 March.
HRW urged Russia, as an occupying power in Crimea, to fully investigate the abduction of the two men and the allegations of torture.
"The investigation should include the alleged involvement of officers from Berkut, the Ukrainian riot police force, which was dissolved by Ukraine's Minister of Interior in February 2014 but remains active in Crimea," HRW said.
"For weeks irregular armed units have been allowed to run amok on the Crimean peninsula without any apparent legal authority or accountability, and it's led to insecurity, arbitrary detentions, abductions, and torture," according to Hugh Williamson, Europe and Central Asia director at Human Rights Watch.
"Whether these armed units, and those working with them, answer to Crimean authorities, to Moscow, or both, they are acting illegally, and the authorities need to call an immediate halt to their abuses," Williamson continued.
The pro-Euromaidan protests were sparked in Ukraine's capital Kiev after former President Viktor Yanukloich spurred a trade and investment deal with Europe, in favour of tighter collaborations with Russia's Vladimir Putin.
HRW already documented the involvement of "self-defence" units in Crimea in the abductions of at least four other activists from the Euromaidan movement.
According to local media freedom groups at least nine people, including six military officers and three activists, remain missing.
The groups allege that self-defence units and paramilitary forces were involved in the abductions.