Fourteen bodies with shots to the head were conveyed to a nearby healing facility in Benghazi on Friday.
The unidentified bodies were allegedly found on Thursday and taken by individuals from the Red Crescent to a therapeutic office in Libya's second city.
UN emissary to Libya Martin Kobler said he was "totally stunned and frightened" by the killings, calling them an "atrocity" in a Twitter post.
No gathering has so far guaranteed obligation regarding the executions.
Libya has been plagued by disorder after previous pioneer Muammar Gaddafi was toppled amid the 2011 political turmoil.
Rough fights proceed amongst aggressors and the powers of Major General Khalifa Haftar since the Libyan general dispatched operation "Nobility" against activist gatherings in Benghazi right around two years back.