As many as 13,000 people were hanged in five years at a notorious Syrian government prison near Damascus, Amnesty International said Tuesday, accusing the regime of a “policy of extermination.”

Titled “Human Slaughterhouse: Mass hanging and extermination at Saydnaya prison,” Amnesty’s damning report is based on interviews with 84 witnesses, including guards, detainees, and judges.

It found that 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.

Most of the victims were civilians believed to be opposed to the government of President Bashar al-Assad.

“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 practices amounted to war crimes and crimes against humanity, but were likely still taking place.

Thousands of prisoners are held in the military-run Saydnaya prison, one of the country’s largest detention centres located 30 kilometres north of Damascus.

Amnesty accused the Syrian government of carrying out a “policy of extermination” there by repeatedly torturing detainees and withholding food, water, and medical care.

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.