Syrian authorities have killed at least 13,000 people since the start of the 2011 uprising, in mass hangings at a prison north of Damascus known to detainees as "the slaughterhouse", Amnesty International has said.
In a new report covering the period from 2011 to 2015, Amnesty said 20-50 people were hanged each week at Saydnaya Prison in killings authorised by senior Syrian officials, including deputies of President Bashar Assad, and carried out by military police.
The report referred to the killings as a "calculated campaign of extrajudicial execution".
Amnesty has recorded at least 35 different methods of torture in Syria since the late 1980s, practices that only increased since 2011, said Lynn Maalouf, deputy director for research at Amnesty's regional office in Beirut.
Other human rights groups have found evidence of massive torture leading to death in Syrian detention facilities.
In a report last year, Amnesty found that more than 17,000 people had died of torture and ill-treatment in custody across Syria since 2011, an average rate of more than 300 deaths a month.
Those figures are comparable to battlefield deaths in Aleppo, one of the fiercest war zones in Syria, where 21,000 were killed across the province since 2011.
"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," Ms Maalouf said.
While the most recent data is from 2015, Ms Maalouf said there was no reason to believe the practice has stopped since then, with thousands more probably killed.
"These executions take place after a sham trial that lasts over a minute or two minutes, but they are authorised by the highest levels of authority", including the Grand Mufti, a top religious authority in Syria, and the defence minister, she said.
Syrian government officials rarely comment on allegations of torture and mass killings.
In the past, they have denied reports of massacres documented by international human rights groups, describing them as propaganda.
The chilling accounts in the Amnesty report came from interviews with 31 former detainees and more than 50 other officials and experts, including former guards and judges.
According to the findings, detainees were told they would be transferred to civilian detention centres but were taken instead to another building in the facility and hanged.
"They walked in the 'train', so they had their heads down and were trying to catch the shirt of the person in front of them," Hamid, a former detainee, told Amnesty.
"The first time I saw them I was horrified. They were being taken to the slaughterhouse."
Another former detainee, Omar Alshogre, told The Associated Press the guards would come to his cell, sometimes three times a week, and call out detainees by name.
He said a torture session would begin before midnight in nearby chambers that he could hear.
"Then the sound would stop, and we would hear a big vehicle come and take them away," said Mr Alshogre, who spent nine months in Saydnaya.
Now 21, he lives in Sweden.
Speaking from Stockholm, he described how he was forced to keep his eyes closed and his back to the guards while they abused or suffocated a cellmate.
The body often would be left behind, or there would be a pool of blood in the cell for other prisoners to clean up.
"We can tell from the sound of the prisoner as he dies behind us," said Mr Alshogre, who at 17 moved among nearly 10 detention centres in Syria for two years before he was taken to Saydnaya.
"He dies a metre away. I don't see anything, but I see with my ears."
Mr Alshogre survived nine months in the prison, paying his way out in 2015 - a common practice.
He suffered from tuberculosis and his weight fell to 35 kilograms (77lbs).
Two cousins detained with him in western Syria did not survive, dying a year apart in a military intelligence detention centre.
The younger one died in Mr Alshogre's arms, deprived of food and so weak he was unable to walk to the bathroom on his own.
Still, Mr Alshogre said nothing could have prepared him for Saydnaya.
At one point he was called out by his guards "for execution", he said.
He was brought before a military trial and told not to raise his gaze at the judge, who asked him how many soldiers he had killed.
When he said none, the judge spared him.
Death in Saydnaya was always present "like the air," he said.
Once when he was deprived of food for two days, a cellmate handed him his food ration and died days later.
"This is someone who gave me his life," he said.
Another cellmate died of diarrhoea, also common in the prison.
"Death is the simplest thing. It was the most hoped for because it would have spared us a lot: hunger, thirst, fear, pain, cold, thinking," Mr Alshogre added.
"Thinking was so hard. It could also kill."