Amnesty International accused Tuesday Syria's government of hanging up to 13,000 people at a notorious prison over five years in a "policy of extermination", two weeks before planned peace talks.
The damning report, titled "Human Slaughterhouse: Mass hanging and extermination at Saydnaya prison" near Damascus, goes into excruciating detail about the gruesome ritual of mass hangings between 2011 and 2015.
At least once a week, up to 50 prisoners were taken out of their 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 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."
Amnesty said the mass executions amounted to war crimes and crimes against humanity, but were likely still taking place.
Hamid, a former army officer who was jailed in 2012, told Amnesty he was simultaneously horrified and relieved when he saw prisoners being taken to be hanged.
"I felt happy that their suffering would come to an end."
In comments published Tuesday, Assad insisted that "defending" his country in a time of war was more important than a potential case against his government at the highest UN court in The Hague.
"We have to defend our country by every mean, and when we have to defend it by every mean, we don't care about this court, or any other international institution," he said.
Amnesty's report comes just two weeks before a new round of talks is due to take place in Switzerland aimed at putting an end to nearly six years of civil war.
"The upcoming Syria peace talks in Geneva cannot ignore these findings. Ending these atrocities in Syrian government prisons must be put on the agenda," said Lynn Maalouf, deputy director for research at Amnesty's Beirut office.
Thousands of prisoners are held at the military-run Saydnaya prison, 30 kilometres (18 miles) north of Damascus, one of Syria's largest detention centres.
Amnesty accused Syria's government of carrying out a "policy of extermination" there by repeatedly torturing detainees and withholding food, water and medical care.
"All you see is blood: your own blood, the blood of others," Salam, a lawyer from Aleppo who was held in Saydnaya from 2012 to 2014, was quoted as saying.
Prisoners were raped or forced to rape each other, and guards would feed detainees by tossing food onto cell floors, which were often covered in dirt and blood, Amnesty said.
A twisted set of "special rules" governed the facility: detainees were not allowed to speak and were forced to 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 number one -- how many? Room number two -- how many?' and on and on," said Nader, a former detainee whose name was changed in the Amnesty report.
After one fierce day of beating, 13 people died in a single wing of the prison, he said.
"If you put your ears on the floor, you could hear the sound of a kind of gurgling," said Hamid, the military officer.
"We were sleeping on top of the sound of people choking to death. This was normal for me then," he told Amnesty.
The watchdog has previously said that more than 17,700 people were estimated to have died in government custody in Syria since the conflict erupted in March 2011.
That figure did not include the up to 13,000 people executed in Saydnaya.
"The horrors depicted in this report reveal a hidden, monstrous campaign, authorised at the highest levels of the Syrian government, aimed at crushing any form of dissent within the Syrian population," said Maalouf.
"The cold-blooded killing of thousands of defenceless 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.
Amnesty said it gave the names of 87 prison officials and guards responsible for the atrocities to unspecified "bodies capable of conducting credible investigations" into the killings.
A UN investigation last year accused Assad's government of carrying out a policy of "extermination" in its jails.
More than 310,000 people have been killed in Syria since the civil war began.