The London-based Syrian Network for Human Rights said in a report torture of those who oppose or intend to oppose the regime of President Bashar Assad is common.
"Torture is one of many instruments the regime always used to punish his [opponents] and was a common knowledge to all Syrians, but indeed what it's impossible for the human mind to comprehend is the torture of the children by the Assad regime, which exceeded any vicious description," the group said in a report, accompanied by videos of incidents at prisons.
The victims included even a 5-month-old baby, Afaf Assaraqbi, the report said.
It's difficult to determine the number of detainees in Syria, where the United Nations has said at least 7,500 people have died in the year-long crackdown on anti-government protests, the Los Angeles Times reported.
Neil Sammonds, a researcher with Amnesty International, said human rights groups have obtained names of about 17,000 detainees but said that likely accounts for only half of those actually held.
Amnesty International said in a February report it scrutinized the cases of 88 prisoners, including 10 children, believed to have died in detention. In at least 52 cases, torture or other mistreatment was considered to have caused or contributed to deaths.