The Assad regime has executed up to 13,000 people, mostly civilians in mass hangings since 2011, Amnesty International said Tuesday.
The London-based rights group said the executions had been carried out in the notorious Saydnaya military prison near Damascus from September 2011 and December 2015.
"The victims are overwhelmingly ordinary civilians who are thought to oppose the government," Amnesty said in a report.
The watchdog, which based its report on interviews with former detainees there, prison employees, judges and others, said the victims had been "extrajudicially executed in mass hangings, carried out at night and in the utmost secrecy".
"Many other detainees at Saydnaya Military Prison have been killed after being repeatedly tortured and systematically deprived of food, water, medicine and medical care. The bodies of those who are killed at Saydnaya are buried in mass graves," it said.
Syria has been locked in a devastating civil war since early 2011, when the Bashar al-Assad regime cracked down on pro-democracy protests with unexpected ferocity.
One month ago, a ceasefire deal brokered by Turkey and Russia went into effect throughout war-torn Syria.
