At least 13,000 people have been hanged at a Syrian prison known as "the slaughterhouse" in a crackdown on dissent by the regime of Bashar al-Assad, a shocking new report by Amnesty International has revealed.

Amnesty's report, Human slaughterhouse, covers the period from 2011 to 2015, when up to fifty people were hanged each week at Saydnaya Prison, located north of Damascus. However, the watchdog says there are "strong reasons to believe that this routine is still ongoing today."

The mass killings, referred to in the report as "calculated campaign of extrajudicial execution", were authorized by senior Syrian officials and carried out by military police. The victims, most of them civilians, were believed to be opposed to the government.

According to accounts taken from 84 witnesses, including security guards, detainees, judges and lawyers, prisoners were told they would be transferred to civilian detention centers.

"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. The first time I saw them, I was horrified. They were being taken to the slaughterhouse," Hamid, a former detainee, told Amnesty.

However, detainees were instead taken instead to another building in the facility, where they were beaten and then hanged.

"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 the hangings.

The bodies were then taken away by the truckload to be secretly buried in mass graves, according to the report, and the families of the victims were given no information about their fate.

According to the report, prior to their execution, the prisoners were given a fake trial by "a so-called Military Field Court."

"The judge will ask the name of the detainee and whether he committed the crime. Whether the answer is yes or no, he will be convicted ... This court has no relation with the rule of law. This is not a court," a former judge said, according to Amnesty.

Survivors of the "human slaughterhouse" also provided horrific testimonies about life inside the prison, revealing that many of the detainees were raped or forced to rape other prisoners. Prisoners were also forbidden to make any sounds, and simply looking at the guards was punishable by death.

"Torture and beatings are used as a regular form of punishment and degradation, often leading to life-long damage, disability or even death," reads the report. "The cell floors are covered with blood and puss from prisoners' wounds. The bodies of dead detainees are collected by the prison guards each morning, around 9am."

In light of the report, Amnesty is calling on the international community, notably the UN Security Council, to "take immediate and urgent action, to put an end to this suffering."

"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," Lynn Maalouf, the deputy director for research at Amnesty's Beirut office, said in a statement.

According to UN estimates, about 400,000 people have been killed since the start of the Syrian civil war in 2011, and another 4.8 million have fled the country. The United Nations High Commissioner for Refugees (UNHCR) has said another 6.6 million are internally displaced within Syria.