Nearly 850 people have died in Syrian government prisons and security forces facilities this year, activists say.
The UK-based Syrian Observatory for Human Rights said 15 children and six women were among the victims.
They lost their lives as a result of torture, executions, maltreatment and poor conditions, it added.
The Observatory - which relies on a network of activists, medical and military sources in Syria - said the number of deaths could be even higher.
It said some 18,000 people among those held by the government in the past three years had disappeared, and many were feared dead.
"The number of victims is increasing because there are no measures being taken to deter the regime," said the Observatory's director, Rami Abdul Rahman.
The Observatory said that in the 847 cases it had documented up until 13 May, the victims' families had been notified by the Syrian authorities.
The government has denied that its security personnel are engaged in the widespread torture, mistreatment and execution of detainees.
Meanwhile, Physicians for Human Rights says government forces have been responsible for 90% of the confirmed assaults on healthcare facilities in Syria, challenging claims by officials that rebels are to blame.
The US-based advocacy group published an interactive map which it said represented the most comprehensive study of attacks on medical workers, hospitals and clinics.
The map links to reports, videos and photographs of the 150 attacks on 124 separate medical facilities documented by PHR since the uprising began in March 2011.
At least 468 civilian medical personnel have reportedly been killed.
"Syria is among the worst examples of targeting medical care as a weapon of war, and we must not allow these rampant abuses to become the new norm in conflict," Donna McKay, the executive director of Physicians for Human Rights, told the New York Times.