Since 2011 the Syrian government has hanged up to 13,000 prisoners following "sham trials" according to a new report
The Syrian government has executed up to 13,000 prisoners at a prison near Damascus since 2011, according to Amnesty international.
The Human rights organisation said systematic torture has also been carried out at the military jail, which is referred to by detainees as "the slaughterhouse."
Some inmates were allegedly executed after "sham trials" lasting no more than a couple of minutes - and the killings were often authorised by senior Syrian officials, including deputies of President Bashar al Assad.
According to the Amnesty figures, groups of up to 50 people were taken out of their cells at the Saydnaya prison and hanged on a regular basis.
The organisation said the hangings occurred once or twice a week between 2011 and 2015.
It said that in five years, as many as 13,000 people - most of them civilians believed to be opposed to the government - were hanged in secret at the jail.
According to the report, the Syrian government is deliberately inflicting inhuman conditions on detainees at the prison through repeated torture and the systematic deprivation of food, water, medicine and medical care.
The organisation said the practices amount to war crimes and crimes against humanity - and is calling for a UN investigation.
Its report said: "The victims are overwhelmingly civilians who are thought to oppose the government. Many other detainees have been killed after being repeatedly tortured and systematically deprived of food, water, medicine and medical care."
Colm O’Gorman, Executive Director of Amnesty International Ireland 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," he said.
"Russia and Iran, the government’s closest allies, must press for an end to these murderous detention policies."
Mr O'Gorman said ending the atrocities must be top of the agenda at the upcoming upcoming Syria peace talks in Geneva.
"The UN must immediately carry out an independent investigation into the crimes being committed at Saydnaya and demand access for independent monitors to all places of detention,” he said.
The report said executions were often carried out in secret. Those killed were buried at mass graves outside the capital, but families were never informed of their fate.
Amnesty interviewed more than 30 ex-inmates at the prison and dozens of other officials and experts, including former guards and judges.
Omar Alshogre was spared execution during his nine months at the jail and now lives in Sweden. One of his cousins died in his arms because he was so deprived of food, while another of his cellmates died of diarrhoea - a common occurrence inside the prison.
He said: "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."
In the past, Syrian government officials have rejected similar reports of torture and extrajudicial killings - describing such allegations as "propaganda."
Amnesty is urging the Syrian state's backers, including Russia and Iran, to condemn the "extermination policies and do what is in their power to bring them to an end."
The group said the findings of the report are based on an intensive investigation involving first-hand interviews with 84 witnesses that included former Saydnaya guards and officials, detainees, judges and lawyers - as well as national and international experts on detention in Syria.