Twice a week, on Monday and Wednesday in the black of night, prisoners were woken in silence.
A list of names were called and they were told they were headed to another facility in the city, which was good news for many.
Instead, the men were piled into trucks known as "meat fridges" and taken to an execution room. After one minute of standing in front of a "judge", a noose was placed around their necks in the first sign that this was the moment of their death.
"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," said a former judge who witnessed such events.
Former military officer Hamid, who was being detained on the floor above, said he would hear the nightmarish noises from below.
"If you put your ears on 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.
Horrific news of the previously unreported mass executions has been revealed overnight by Amnesty International in a 48-page report based on interviews with 84 former detainees, judges, guards and human rights experts from the Saydnaya Military Prison in Syria.
The human rights group said up to 13,000 people, most of whom were civilians, were systematically killed in groups of as many 50 twice a week over the past five years in executions that were sanctioned at the "highest levels" of the Syrian government.
Amnesty International's regional director based in Beirut, Lynn Maalouf, said 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.
"We demand that the Syrian authorities immediately cease extrajudicial executions and torture and inhuman treatment at Saydnaya Prison and in all other government prisons across Syria. Russia and Iran, the government's closest allies, must press for an end to these murderous detention policies," she said.
'The end of life'
The graphic report makes for shocking reading and details the horrific conditions inside the all-male military prison located 30km north of Damascus.
Former detainees spoke of the horrendous conditions in which they were kept including being deprived of food, water and basic medicine while being tortured and forced to remain silent.
Reconstructions of the site, which remains a "black hole" for journalists, show a "red" and "white" building. One housed political dissidents including journalists, doctors, students and activists, the other was used to hold former military officials and members of the regime.
Twice a week, prisoners were told they were heading to better conditions and sent to the notorious "meat fridges" before their perfunctory trial at a military field court.
High-school student Omar told Amnesty he could hear prisoners being beaten with different instruments, which he could differentiate in the darkness based on the sounds they made.
"First we were thinking the people were being released or taken to the civilian prisons. But at midnight, we heard the sound of torture again, and we thought they were dying, because the sound of the torture was so strong. They were beating them in a monstrous way."
Former military officer Hamid said he could hear the hangings from above the execution room where the sound of a plank of wood being pulled was followed by noises of a strangling.
"If you put your ears on 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."
Others told of how their brains started to develop in a "strange way" as a result of torture, with depression, psychosis, infection and diseases like tuberculosis common.
Former prisoner Wael who was held there from 2012 to 2014 said: "We didn't think about what we were doing - we just reached the state of barbarity, and entered it, not even thinking. Everything we did was part of the battle of survival. It's a real war, and, in the end, if you refuse to fight it, you will die."
Amnesty International is now calling for an immediate end to extrajudicial killings conducted without charges, legal aid or trial. It also wants the UN Human Rights council to investigate violations occurring there.
"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. Those responsible for these heinous crimes must be brought to justice," Maalouf said.
Syrian President Bashar Al Assad has recently regained the upper hand in the Syrian civil war that has been raging for since 2011, claiming an estimated 400,000 lives and creating 10 million refugees.
A previous report by Amnesty International said 17,000 people have died in prisons across the country, in addition to the 13,000 named in the Human Slaughterhouse report.