As many as 13,000 people, most of them civilian opposition supporters, have been executed in secret at a prison in Syria, Amnesty International says.
A new report by the human rights group alleges that mass hangings took place every week at Saydnaya prison between September 2011 and December 2015.
The alleged executions were authorised at the highest levels of the Syrian government, Amnesty said.
The government has previously denied killing or mistreating detainees.
However, United Nations human rights experts said a year ago that witness accounts and documentary evidence strongly suggested that tens of thousands of people were detained and "deaths on a massive scale" occurred in custody.
Amnesty interviewed 84 people, including former guards, detainees and prison officials for its report.
It alleged that every week, and often twice a week, groups of between 20 and 50 people were executed in total secrecy at the facility, just north of Damascus.
Before their execution, detainees were brought before a "military field court" in the capital's Qaboun district for "trials" lasting between one and three minutes, the report said.
A former military court judge quoted by Amnesty said detainees would be asked if they had committed crimes alleged to have taken place.
"Whether the answer is 'yes' or 'no', he will be convicted... This court has no relation with the rule of law," he said.
According to the report, detainees were told on the day of the hangings that they would be transferred to a civilian prison - but then taken to a basement cell and beaten over the course of two or three hours.
Then in the middle of the night they were blindfolded and moved to another part of the prison, where they were taken into a room in the basement and told they had been sentenced to death just minutes before nooses were placed around their necks, the report added.
The bodies of those killed were then loaded onto lorries, and transferred to Tishreen military hospital in Damascus for registration and burial in mass graves located on military land, the report said.
On the basis of evidence of the testimony of its witnesses, Amnesty estimates that between 5,000 and 13,000 people were executed at Saydnaya over five years.
The Amnesty report quoted a former judge who saw the hangings: "They kept them [hanging] there for 10 to 15 minutes. Some didn't die because they are light. For the young ones, their weight wouldn't kill them. The officers' assistants would pull them down and break their necks."
'Hamid', a former military officer who was detained at Saydnaya, told Amnesty that if he put his ear to the floor, "you could hear the sound of a kind of gurgling".
"This would last around 10 minutes ... We were sleeping on top of the sound of people choking to death. This was normal for me then," he said in the report.
Although it does not have evidence of executions taking place since December 2015, the group said it had no reason to believe they had stopped and that thousands more were likely to have died.
The practices amounted to war crimes and crimes against humanity, Amnesty said.
It noted that death sentences have to be approved by the grand mufti and by either the defence minister or the army's chief of staff, who are deputised to act on behalf of President Bashar al-Assad.
The human rights group said it contacted the Syrian authorities about the allegations in early January but received no response.
Last August, Amnesty reported that an estimated 17,723 people had died in custody as a result of torture and the deprivation of food, water and medical care between March 2011 - when the uprising against President Assad began - and December 2015. That figure did not include those allegedly hanged at Saydnaya.