The New York-based Human Rights Watch said the frequency of torture in Syrian detention centers could constitute crimes against humanity and called for international action.
"The systematic patterns of ill-treatment and torture that Human Rights Watch documented clearly point to a state policy of torture and ill-treatment and therefore constitute a crime against humanity," the group said.
It called on the United Nations Security Council to refer the situation in Syria to the International Criminal Court in the Hague and to impose sanctions on those who practice or oversee torture.
The Syrian government did not immediately comment on the allegations.
Human Rights Watch released the findings Tuesday in a 78-page report based on interviews with more than 200 army defectors and former detainees since the start of the uprising against President Bashar Assad in March, 2011.
It gives the locations of 27 detention centers in cities across Syria that use torture and the names of some of the facilities' commanders.
The group said each of the centers and techniques was mentioned by multiple people interviewed, and that there are probably other facilities that the group has not been able to document.
Many former detainees reported being held in overcrowded facilities where torture by guards and interrogators was commonplace. Interviewees reported more than 20 different torture methods, including beatings with sticks and cords, squeezing fingers with pliers, attacks with electricity and acid and sexual assault and humiliation.
Syria's uprising began with protests calling for political change. Assad's security forces swiftly cracked down, dispatching troops and pro-government thugs to beat and kill protesters. The conflict has changed into an insurgency as many in the opposition have taken up arms to protect themselves and attack government troops.
Anti-regime activists put the death toll more than 14,000.
The Syrian government denies the uprising seeks political reform and blames it instead on armed gangs and terrorists backed by foreign powers that want to weaken Syria.