Nearly 18,000 people have died in Syrian regime prisons from torture or other ill-treatment since 2011, rights group Amnesty International said Thursday.
The London-based watchdog estimated that 17,723 people had died in custody between March 2011, when pro-democracy protests erupted, and December 2015.
More than 300 deaths occurred on average each month, it said.
Amnesty's report recounts the experience of dozens of torture survivors who spoke of abuse and inhumane conditions in jails operated by security intelligence agencies, as well as in the notorious Saydnaya Military Prison on the outskirts of Damascus.
"They treated us like animals. They wanted people to be as inhuman as possible," a former prisoner named Samer told Amnesty.
Most of the survivors said they witnessed prisoners dying in custody. Some said they had being held alongside dead bodies.
They said they suffered from severe beatings, rape, electric shock and the pulling of toenails and fingernails. They also spoke of being scalded with hot water and burned by cigarettes.
"For decades, Syrian government forces have used torture as a means to crush their opponents," said Philip Luther, the director of Amnesty International's Middle East and North Africa Programme.
"Today, it is being carried out as part of a systematic and widespread attack directed against anyone suspected of opposing the government in the civilian population and amounts to crimes against humanity," he said.
Luther called on the international community - especially Russia and the United States, which are co-leading peace talks on Syria - to bring prison abuse to the top of the agenda in their discussions with the country's warring factions.