18,000 people died under government custody in Syria in 2011-15 an Amnesty International report says, alleging beatings and rape in prisons.
Amnesty says file contains interviews with 65 “torture survivors”, who have described appalling abuse in jails and detention centres.
The human rights group urges the world community to pressure Damascus to end the use of torture.
The Syrian government has repeatedly denied such allegations.
“It breaks the human”: Torture, disease and death in the prisons of Syria – was released by Amnesty on Thursday.
The group estimates that more than 17,723 people died in custody across Syria between March 2011, when the uprising against President Bashar Assad started, and December 2015.
That is more than 300 a month, or about 10 people every day, Amnesty says.
Detainees tend to be subjected to severe beatings by prison guards upon after their arrival at a penitentiary.
This maltreatment is called a “welcome party”.
This is frequently commonly followed by “security checks”, during which women in particular reported being raped and sexually assaulted by male guards, Amnesty says.
“They treated us like creatures. They needed folks to be as inhuman as possible,” detainee Samer told Amnesty.
I never envisioned humanity would reach this kind of low level.. they would have had no problem killing us right there and then,” he said.
Another prisoner, Ziad (not his actual name) described how seven people died in one day after the ventilation stopped working at an intelligence agency detention centre.
“They started to kick us to see who was living and who was not,” Ziad said.
“Today, it has been carried out as part of a systematic and widespread attack directed against anyone suspected of opposing the government in the civilian inhabitants and quantities to crimes against humanity,” he added.
Amnesty and other human rights groups say the claims have to be discussed urgently by the international community, particularly the US and Russia, who co chair peace talks on the Syrian disaster.