Syrian intelligence agencies are running a network of torture centers across the country where detainees are beaten with batons and cables, burned with acid and sexually assaulted, Human Rights Watch said in a report on Tuesday.
The state-sanctioned abuses amounted to crimes against humanity that should be investigated by the International Criminal Court, the New York-based campaign group added.
Its report identified 27 detention centers it said intelligence agencies have been using since March 2011 when President Bashar al-Assad's government began a crackdown on pro-democracy protests that have grown into an armed revolt.
Syria's government did not immediately respond to the accusations which have been echoed in earlier reports by the United Nations.
Human Rights Watch said tens of thousands of people had been detained by Syria's Department of Military Intelligence, the Political Security Directorate, the General Intelligence Directorate and the Air Force Intelligence Directorate.
The group said it conducted more than 200 interviews with people who said they were tortured, including a 31-year-old man who was detained in the Idlib area in June and made to undress.
"Then they started squeezing my fingers with pliers. They put staples in my fingers, chest and ears. I was only allowed to take them out if I spoke. The staples in the ears were the most painful," he was quoted as saying.
"They used two wires hooked up to a car battery to give me electric shocks. They used electric stun-guns on my genitals twice. I thought I would never see my family again. They tortured me like this three times over three days," he added.
Human Rights Watch said it documented more than 20 torture methods that "clearly point to a state policy of torture and ill-treatment and therefore constitute a crime against humanity".
United Nations human rights investigators have made similar reports.
"Torture is one of the most extensively and best documented of the many awful human rights violations taking place in Syria over the past 15 months," U.N. human rights spokesman Rupert Colville told a news briefing in Geneva on Tuesday.
There has been a "constant stream of very consistent information about widespread systematic use of torture, and of course if it is widespread and systematic, that amounts to a crime against humanity," he said.
A recent U.N. mission to the region interviewed people who had faced severe beatings, electric shocks, cigarette burns, mock executions, and sleep deprivation, he said.
Human Rights Watch called for the U.N. Security Council to refer Syria to the International Criminal Court (ICC) and impose sanctions on officials carrying out abuse.
"The reach and inhumanity of this network of torture centers 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."
Russia - an ally of Syria - and China have already vetoed two council resolutions that condemned Damascus and threatened it with sanctions.
The United Nations has said more than 10,000 people have been killed during the 16-month Syria conflict.