Police have rescued hundreds of boys from a “house of torture” in Nigeria after raiding an Islamic centre where pupils as young as nine were allegedly tortured, sexually abused and shackled to walls.

Officers who stormed a building in the northern city of Kaduna said on Friday that they had found nearly 500 emaciated men and boys, about 100 of whom had been held in chains in its cramped basement, some for several months.

The establishment’s owner and seven suspects described as teachers were arrested after victims told their rescuers that they had been sodomised, starved and subjected to frequent beatings by members of staff. Some may even have died.

Police released a photograph of one boy’s back covered in deep welts.

“We found around 100 students, including children as young as nine, in chains stuffed in a small room,” Yakubu Sabo, the Kaduna state police spokesman, told reporters.