Bangladeshi police detectives on Friday forced the discharge from hospital of three student protest leaders blamed for deadly unrest, taking them to an unknown location.

Asif Mahmud, Nahid Islam and Abu Baker Majumder are all members of Students Against Discrimination, the group responsible for organising recent street rallies against civil service hiring rules.

At least 193 people were killed in the ensuing police crackdown and clashes, according to a count of victims reported by police and hospitals, in some of the worst unrest of Prime Minister Sheikh Hasina's tenure.

The trio were being treated for injuries that they said were caused by torture in earlier police custody at a hospital in the capital Dhaka.

"They took them from us," Gonoshasthaya hospital supervisor Anwara Begum Lucky said. "The men were from the Detective Branch."

She added that she had not wanted to discharge the student leaders but police had pressured the hospital chief to do so.

Islam's elder sister Fatema Tasnim said from the hospital that six plainclothes detectives had taken all three men. (AFP)