A human rights group says it has found documents and physical evidence indicating Syrians were arbitrarily detained and tortured in government prisons in the eastern city of Raqqa.
Rebels overran the city in February and facilitated access to Human Rights Watch.
The group says its researchers visited Raqqa in late April and inspected security facilities that belonged to the government and military intelligence.
HRW found documents, prison cells, interrogation rooms and torture devices in the detention centres.
The evidence appears "consistent with the torture former detainees have described", the New York-based group said in a report.
Raqqa is the first Syrian city under full opposition control.
HRW has been documenting abuses on both sides of Syria's civil war since the beginning of the conflict in March 2011
HRW said its researchers found physical evidence indicating Syrians were tortured in cells, including with a device which former detainees said was used to stretch or bend victims' arms and legs.
The group also found documents indicating Raqqa residents were detained for legal actions like demonstrating or helping the injured.
Rights groups and opposition activists have long claimed that civilians have been detained arbitrarily, tortured and sometimes have disappeared since the uprising against Assad's regime began. HRW's findings appear to be one of the largest finds of physical evidence bolstering those claims.
"The documents, prison cells, interrogation rooms, and torture devices we saw in the government's security facilities are consistent with the torture former detainees have described to us," said Nadim Houry, deputy Middle East director for HRW.
HRW says abuses by the Assad regime remain far more deadly, systematic and widespread, including on civilians with indiscriminate battlefield weapons such as widely banned cluster bombs. But the rights group also says rebel abuses have increased in frequency and scale in recent months.
The conflict has killed at least 70,000 people and forced millions out of their homes to seek shelter in neighbouring countries or in other parts of Syria where fighting has subsided.
In Raqqa, the group's researchers inspected the State Security and Military Intelligence branches and three other detention centres formerly managed by Criminal Security, Political Security and Air Force Intelligence. Government forces abandoned all these institutions that are now controlled by the rebels, the group said.
On the ground floor and in the basement of the State Security facility, HRW found "rooms that appeared to be detention cells", the report said. They also found a pile of documents, including what appeared to be lists of security force members who worked there.
Four former detainees said that officers and guards in the facility tortured them, HRW said.
In one method of torture, the victims were tied to a flat board, sometimes in the shape of a cross. In some cases guards stretched or pulled their limbs or folded the board in half so their face touched their legs.
The group also interviewed five people formerly held by Military Intelligence in Raqqa. They said security services questioned them about lawful activities, such as participating in anti-Assad demonstrations, providing relief assistance to displaced families, defending detainees and providing emergency assistance to injured demonstrators.