At times groups of up to 50 people were taken out of their prison cells at Sadnaya jail for arbitrary trials, beaten and then hanged "in the middle of the night and in total secrecy."
As many as 13,000 people opposing the Syrian regime of Bashar al Assad were hanged in the first five years of the Syria civil war at a notorious prison near Damascus as part of the regime's "policy of extermination", Amnesty International said on Tuesday.
According to an Amnesty's report, at least once a week between 2011 and 2015, groups of up to 50 people were taken out of their prison cells for arbitrary trials, beaten, then hanged "in the middle of the night and in total secrecy."
"Throughout this process, they remain blindfolded. They do not know when or how they will die until the noose was placed around their necks," the rights group wrote in the report titled "Human Slaughterhouse: Mass hanging and extermination at Saydnaya prison."
Thousands of prisoners are held in the military-run Saydnaya prison, one of the country's largest detention centres located 30 kilometres (18 miles) north of Damascus.
The Amnesty's report is based on interviews with 84 witnesses, including guards, detainees, and judges.
Most of the victims were civilians believed to be opposed to the Bashar al Assad's regime.
"They kept them (hanging) there for 10 to 15 minutes," a former judge who witnessed the executions said.
"For the young ones, their weight wouldn't kill them. The officers' assistants would pull them down and break their necks," he said.
Amnesty said the practise amounted to war crimes and crimes against humanity, but were likely still taking place.
Prisoners were raped or forced to rape each other, and guards would feed detainees by tossing meals onto the cell floor, which was often covered in dirt and blood.
The group has previously said that more than 17,700 people were estimated to have died in government custody across Syria since the country's conflict erupted in March 2011.
The figure of 13,000 deaths in a single prison, therefore, is a marked increase.