The group said it discovered solitary confinement cells in the centers as well as torture devices like the cross-shaped "flying carpet," to which detainees would be attached and beaten and bent into painful positions.
Human rights groups estimate at least 25,000 Syrians are in detention for allegedly anti-government marches and protests. The number of detainees escalated sharply after the start of the uprising against President Bashar Assad started in March 2011, The New York Times reported Saturday.
Human Rights Watch investigators in April visited branches of the State Security and Military Intelligence in Raqqa, which came under rebel control in March.
Lama Fakih, a researcher on the visitation, said the team found documents listing case files for local residents and security staff members.
"You could see in each case file the picture of the person who was being investigated, how old he was, where he lived and other things like the types of activities he was engaged in," she said.
She said the team interviewed men who had been held in the centers. The men said they were detained for marching against the government and had been beaten and shocked with electricity.
Notable deaths of 2014 [PHOTOS]