The campaign group, Human Rights Watch (HRW), says thousands of mentally ill people in Nigeria are being abused and kept in chains in care facilities.
Its researchers visited nearly 30 private and state-run centres across the country.
In all but one, they found patients were being chained up.
Staff at the facilities said the practice stops the patients from leaving or harming others.
In recent months, the Nigerian authorities have carried out a series of raids on private religious centres, freeing more than 1,000 people who had been kept in shackles.
HRW is calling for those rescued to be given proper support by the government.