The Syrian government is practicing a widespread policy of state-sanctioned torture, according to a new report from Human Rights Watch.
The organisation says it has interviewed more than 200 former detainees and identified at least 27 detention centres across Syria.
Its details horrific torture methods, enforced disappearances and arbitrary arrests.
The report claims the regime's actions amount to a crime against humanity.
Human Rights Watch wants the regime to be tried at the International Criminal Court (ICC), but correspondents say any such move is likely to be blocked by Russia.
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," he said.
The report, "Torture Archipelago", draws on the testimony of more than 200 former detainees, including women and children, to describe what it says are cases of mistreatment and torture since March 2011.
Human Rights Watch said everyone it interviewed described conditions of extreme overcrowding and a lack of food, while many said they had been forced to sign or fingerprint confessions admitting that they had attended anti-government protests.
The majority of the eyewitnesses also said they had been tortured and several of them claimed to have seen people die from torture in detention.
In one account, a 13 year old-boy described being tortured for three days at a military security branch near Tal Kalakh.
"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."
Eyewitnesses described a wide range of tortures used, including electric shocks, sexual abuse (of both male and female detainees), beatings with batons and whips, hanging from the wrists for long periods and sleep deprivation.
Amer, a 23-year old arrested in Idlib province, 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," he said.
A former intelligence officer is quoted in the report as saying the orders to torture prisoners came directly from security forces who were in touch with President Assad's entourage.
Human Rights Watch would like the UN to send observers to monitor the situation in Syria's detention centres and to refer the issue to the International Criminal Court.
Because Syria has not ratified the Rome Statute, which created the ICC, the court will only have jurisdiction if the Security Council adopts a resolution referring the situation in Syria to the court.
Russia and China have previously blocked Security Council efforts to hold the Syrian regime accountable.
"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."