Human Rights Watch, in an 81-page report based on more than 200 interviews, highlighted grave abuses committed by Syrian forces since March 2011.
Most of the detainees interviewed by the rights groups alleged they were tortured repeatedly through the use of electric shocks, mock executions and sexual assault. The organization said it documented more than 20 different types of torture carried out by Syrian security and intelligence services.
British Foreign Secretary William Hague, in a statement Tuesday, said the level of violence alleged by the rights group was appalling.
"Those responsible for systematic and widespread human rights violations should not delude themselves," he said. "We and our international partners will do everything we can to ensure that they will face justice."
HRW accounts correspond with findings from the United Nations, which added targeted killings of activists and opposition supporters by government forces inside Syria have increased.
Human Rights Watch called on the U.N. Security Council to refer Damascus to the International Criminal Court. The council, however, has been unable to overcome a veto threat by China and Russia, Syria's largest weapons supplier.