GAZIANTEP, Turkey — Syrian security officers hung Muhannad Ghabbash from his wrists for hours, beat him bloody, shocked him with electricity and stuck a gun in his mouth.

Mr. Ghabbash, a law student from Aleppo, repeatedly confessed his actual offense: organizing peaceful antigovernment protests. But the torture continued for 12 days, until he wrote a fictional confession to planning a bombing.

That, he said, was just the beginning.

He was flown to a crammed prison at Mezze air base in Damascus, the Syrian capital, where he said guards hung him and other detainees from a fence naked, spraying them with water on cold nights. To entertain colleagues over dinner, he and other survivors said, an officer calling himself Hitler forced prisoners to act the roles of dogs, donkeys and cats, beating those who failed to bark or bray correctly.

In a military hospital, he said, he watched a nurse bash the face of an amputee who begged for painkillers. In yet another prison, he counted 19 cellmates who died from disease, torture and neglect in a single month.