Tens of thousands of people in Syrian prisons have been tortured and nearly 18,000 have died since March 2011, says a report from Amnesty International.

The report recounts the experiences of thousands of former detainees including 65 torture survivors who describe everything from overcrowding to sleeping among corpses.

Former detainees told Amnesty International the abuse often started from the day they entered prison. The so-called "welcome parties" involved beatings with silicone or metal rods, which were followed by security checks that often involved the sexual assault of female prisoners.

"They treated us like animals. They wanted people to be as inhuman as possible … I saw the blood, it was like a river … I never imagined humanity would reach such a low level … they would have had no problem killing us right there and then," said Samer, a lawyer arrested near Hama.

Lit candles are placed by activists around a placard during a vigil in solidarity with the detainees in Syrian prisons at Aleppo's Salaheddine neighbourhood on Jan. 26, 2015. (Ammar Abdullah/Reuters)

Other prisoners spoke about being forced into a rubber tire or being flogged on the feet during interrogations.

The violence was combined with conditions lacking food, medical care and sanitation.

Overcrowding often required prisoners to sleep in shifts or in a squatting position, the report states.

Suffocation

One detainee said the ventilation in Military Intelligence Branch 235 in Damascus stopped working and seven people died from suffocation.

"They began to kick us to see who was alive and who wasn't. They told me and the other survivor to stand up … that is when I realized that … seven people had died, that I had slept next to seven bodies … I saw the rest of the bodies in the corridor, around 25 other bodies," the man said.

The abuses date back to the start of the Syrian uprising against President Bashar al-Assad in March 2011. The government's harsh crackdown on dissent and the rise of armed opposition groups eventually ignited a civil war that has killed more than 250,000 people, displaced half the country's population and generated more than 4.8 million refugees.

A Syrian detainee arrested during protests against Syrian President Bashar al-Assad's regime is seen in a prison vehicle at the Damascus police leadership building to sign his release papers on July 11, 2012. But a new report reveals many more inmates — 17,723 in total — have died behind bars in the war-torn country. (Khaled Al Hariri/Reuters)

"The catalogue of horror stories featured in this report depicts in gruesome detail the dreadful abuse detainees routinely suffer from the moment of their arrest, through their interrogation and detention behind the closed doors of Syria's notorious intelligence facilities," Philip Luther, director of Amnesty International's Middle East and North Africa Program, said in a news release.

Luther said that torture has been commonly used in Syria for decades, but has become "part of a systematic and widespread attack directed against anyone suspected of opposing the government."

These abuses need to be top of mind during any peace talks with Syria, especially those being led by the United States and Russia, Luther said.

Thousands have disappeared

A key element of the report is new data from the Human Rights Data Analysis Group, which found that 17,723 prisoners have died in custody between March 2011 and December 2015, which is an average of 300 people per month, the report states.

The analytics group and Amnesty International believe that is a conservative figure, because thousands of people have disappeared from Syrian prisons.

A new report from Amnesty International states that the most serious abuses in Syria date to the uprising against President Bashar al-Assad in March 2011.

Before March 2011, Amnesty International had found roughly 45 people per year died in custody in Syria.

The reports have several accounts of the violence in Saydnaya Military Prison, where prisoners begin their sentences in underground cells without heat or blankets.

Salam, a lawyer from Aleppo, described how a prisoner who was a kung fu instructor was beaten to death by guards for training other detainees.

"They beat the trainer and five others to death straight away, and then continued on the other 14. They all died within a week. We saw the blood coming out of the cell," Salam said in the report.

Amnesty International is calling for the international community to press Syria to stop the use of torture and to help provide support for those who have been traumatized by their time in prison.

"The international community must make it a priority to end this kind of appalling and entrenched abuse. For years Russia has used its UN Security Council veto to shield its ally, the Syrian government, and to prevent individual perpetrators within the government and military from facing justice for war crimes and crimes against humanity at the International Criminal Court. This shameful betrayal of humanity in the face of mass suffering must stop now."