A report by New York-based NGO Human Rights Watch has revealed widespread torture by the government.
HRW interviewed more than 200 former detainees, including women and children who were held in an "archipelago of torture centres".
The group says the torture amounts to crimes against humanity and called for UN observers to examine Syrian detention centres and refer the issue to the International Criminal Court.
The report contains horrific details of torture methods, including an account by a 13-year old boy.
"They said, 'You pigs, you want freedom?' They interrogated me by myself.. they electrocuted me on my stomach, with a prod. I fell unconscious.
"When they interrogated me the second time, they beat me and electrocuted me again. The third time they had some pliers, and they pulled out my toenail."
23-year old Amer was arrested in Idlib province and described being subjected to the basat al-reeh, or "flying carpet" and the dulab, or "tire method".
"They put me on a flying carpet - I was lying on my back, tied to a board, and they lifted my head and legs. All this time I was undressed. They wrapped wires around my penis and turned on the electricity... I passed out.
"When I regained consciousness they were pushing my legs and hands into a tyre. My entire body was blue from beatings,"
"The reach and inhumanity of this network of torture centres are truly horrific," Ole Solvang, emergencies researcher at Human Rights Watch said.
"Russia should not be holding its protective hand over the people who are responsible for this."
UK Foreign Secretary William Hague said the report "should act as a clear warning".
"Those responsible for systematic and widespread human rights violations should not delude themselves: we and our international partners will do everything we can to ensure that they will face justice,"
