Welcome to hell — or, as it is known officially, “the human slaughterhouse.”

Once a week over a four-year period as many as 50 detainees were retrieved from their cells, blindfolded and hanged inside a Syrian prison's confines, according to a chilling report published Tuesday.

Up to 13,000 prisoners met this inhumane fate from 2011 to 2015, says Amnesty International in Human Slaughterhouse: Mass Hangings and Extermination at Saydnaya Prison, Syria. “Saydnaya is the end of life — the end of humanity,” said Abu Muhammed, a former guard quoted in the report.

Men were given mini, two-minute trials and, with the blessing of senior Syrian officials, hanged. Detainees were tricked into thinking they were being transferred to another detention centre, instead dropped off at a special killing building to be “quietly” slaughtered.

“They kept them (hanging) there for 10 to 15 minutes,” said an ex-judge. “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.”

Starvation and suffocation were regular torture tactics employed by prison officials, says ex-detainee Omar Alshogre, who spoke to the Associated Press. Around midnight multiple times a week the torture sessions would begin just feet from his cell, the horror audible then and during the execution nearby.

“We can tell from the sound of the prisoner as he dies behind us. He dies a meter away. I don’t see anything, but I see with my ears,” Alshogre told the news agency before adding prisoners were tasked with cleaning up pools of blood the next day.

Amnesty's report calls for dramatic change, labeling the mass killings a “calculated campaign of extrajudicial execution.” And the unimaginable cruelty is likely ongoing, says Lynn Maalouf of Amnesty, a regional director based in nearby Beirut.

“The horrors depicted in this report reveal a hidden, monstrous campaign, authorized at the highest levels of the Syrian government, aimed at crushing any form of dissent within the Syrian population,” Maalouf said.

In recent years, other rights groups have revealed similar findings pertaining to the conditions inside detention facilities in Syria, while Amnesty found that more than 17,000 people in custody have died via torture and ill-treatment since 2011.

Amnesty International​ claims that up to 13,000 have been hanged since 2011 at a prison near Damascus https://t.co/Q3l2j98mVS — Sky News (@SkyNews) February 7, 2017

That average rate — more than 300 deaths per month in Syrian prisons — is comparable to battlefield deaths in Aleppo, one of the fiercest war zones in Syria, where 21,000 were killed across the province since 2011.

— with files from the Associated Press