More than 17,000 people have died in Syrian government detention facilities since the start of the 2011 uprising against President Bashar Assad, Amnesty International has said.
The international human rights group said in a new report that many other people have been tortured.
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.
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.