Twelve civilians, including four members of one family, from the same Damascus district were killed under torture in prison after their arrest, the Syrian Observatory for Human Rights reported on Thursday.
Families received the identity cards of the victims from the security forces on Wednesday night, according to the Britain-based watchdog.
The men were all from Nahr Aisha district in the embattled south of the capital, which has seen regular raids and arrests by government troops that have resulted in the imprisonment, torture and killing of other residents, it said.
The Local Coordination Committees, a network of opposition activists on the ground, also reported the deaths and identified the men by name.
The Observatory, which collects reports from a wide network of activists, rights lawyers and medics in civilian and military hospitals, has documented hundreds of cases in which detainees have been tortured to death and many others in which torture led to permanent disability.
It estimates that "tens of thousands" of Syrians are being held in prisons throughout the country.
"There is no exact number of detainees because we don't know what happens after their arrest. Over 200,000 people have been taken prisoner, but we don't know how many were killed," Observatory director Rami Abdel Rahman told AFP.
Human Rights Watch last week demanded unfettered access to Syrian prisons after a prominent peace activist died in custody and another was feared dead.