As many as 13,000 people were hanged in five years at a notorious Syrian prison near Damascus, Amnesty International has said, accusing the regime of a "policy of extermination".
Titled "Human Slaughterhouse: Mass hanging and extermination at Saydnaya prison," Amnesty's damning report, released on Tuesday, 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."
The report said: "Throughout this process, they remain blindfolded. They do not know when or how they will die until the noose was placed around their necks."
Most of the victims were civilians believed to be opposed to the regime of 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 practice amounted to war crimes and crimes against humanity, and were probably still taking place.
Thousands of prisoners are held in the military-run Saydnaya prison, one of the country's largest detention centers located 30km north of Damascus.
Amnesty accused the Syrian regime of carrying out a "policy of extermination", repeatedly torturing detainees and withholding food, water, and medical care.
Prisoners were also raped or forced to rape each other, and guards would feed detainees by tossing meals on to the cell floor, which was often covered in dirt and blood, the report said.
'Hidden, monstrous campaign'
A twisted set of "special rules" governed the facility: detainees were not allowed to speak and must assume certain positions when guards entered their cells.
"Every day there would be two or three dead people in our wing ... I remember the guard would ask how many we had. He would say, 'Room No 1 - how many? Room No 2 - how many?' and on and on," said Nader, a former detainee whose name has been changed.
After one fierce day of beating, Nader said, 13 people died in a single wing of the prison.
One former military officer said he could hear "gurgling" as people were hanged in an execution room below.
"If you put your ears on the floor, you could hear the sound of a kind of gurgling," said Hamid, who was arrested in 2011.
"We were sleeping on top of the sound of people choking to death. This was normal for me then," he told Amnesty.
According to the report, the bodies were taken away by the truckload to be secretly buried in mass graves. Their families were given no information about their fate.
Amnesty has previously said that more than 17,700 people were estimated to have died in regime custody across Syria since the country's conflict erupted in 2011.
The figure of 13,000 deaths in a single prison, therefore, is a marked increase.
"The horrors depicted in this report reveal a hidden, monstrous campaign, authorized at the highest levels of the Syrian regime, aimed at crushing any form of dissent within the Syrian population," said Lynn Maalouf, deputy director for research at Amnesty's Beirut office.
"The cold-blooded killing of thousands of defenseless prisoners, along with the carefully crafted and systematic programmes of psychological and physical torture that are in place inside Saydnaya prison cannot be allowed to continue," she said.
An investigation by the United Nations last year accused Assad's regime of a policy of "extermination" in its jails.
The UN estimates that more than 400,000 people have been killed and millions have fled their homes since the conflict began with anti-Assad protests.