The international human rights group said in a new report that many other people have been tortured.

A catalogue of evidence reveals 19 prisoners were beaten to death after guards found one teaching martial arts skills; prisoners suffocated in overcrowded cells; inmates had their fingernails and toenails pulled out; and guards used a variety of torture methods during interrogations, including forcing male prisoners to rape each other or risk being shot dead.

Amnesty International estimates that more than 17,723 people died in custody in Syria between March 2011 and the end of 2015.

“With tens of thousands of people forcibly disappeared in detention facilities across Syria, the real figure is likely to be even higher,” the report said.

It said common methods of torture included forcibly contorting the victim’s body into a tyre and flogging on the soles of the feet.

The authorities also used electric shocks, rape and sexual violence, the pulling out of fingernails or toenails, scalding with hot water and cigarette burns.

One survivor, named only as Omar S, recalled being made to watch a prisoner being forced to rape another.

“ The guard stripped all of us. And he chose two of us, one of us huge, and the other very small.

“He told the two to come to him. He asked them to turn in a circle, to show him their bodies. Then he ordered the bigger one to rape the smaller one. Because of the torture, and the situation, he couldn’t, even if he had tried. The guard told him he had to do it or he would die.”

The report was based on interviews with 65 torture survivors who described witnessing several deaths in custody. Detainees are frequently subjected to a beating upon arriving. One witness said the weak were targetted on arrival to prison: “It felt like the purpose was death, some form of natural selection — to get rid of the weak as soon as they arrive.

“They first asked my friend and he said, ‘Yes, I have breathing problems — I have asthma.’ They started beating him until he died, right there in front of me.”

Another detainee, Said, a pro-democracy activist, said he was hung by one hand and blindfolded. “While I was hanging... they used an electroshock baton to hit my penis. Then they took the electroshock device and inserted it into my anus and switched it on. This was my first experience of rape. Then one of the guards asked for my face to be uncovered and I saw my father there. He had witnessed all of it.”

“For decades, Syrian government forces have used torture as a means to crush their opponents,” said Amnesty’s Middle East and North Africa director Philip Luther. “Those responsible for these heinous crimes must be brought to justice.”