Nigerian Police say nearly 500 men and boys have been rescued from a building in the northern city of Kaduna, where the detainees were allegedly sexually abused and tortured.
They say children as young as five were among those in chains at what was thought to be an Islamic school.
Kaduna police chief Ali Janga told the BBC the building was raided after a tip‑off about suspicious activity.
He described it as a "house of torture" and a place of human slavery.
Eight suspects, most of them teachers, were arrested. The police chief said the detainees ‑ some with injuries and starved of food ‑ were overjoyed to be freed.
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