KATSINA, Nigeria (Reuters) - Police freed about 500 men and boys from an Islamic school in northern Nigeria on Wednesday, many of whom had been chained to walls, molested and beaten, police sources said.

FILE PHOTO: Police officers, journalists and a traditional chief stand next to people who were rescued by police in Sabon Garin, in Daura local government area of Katsina state, Nigeria October 14, 2019. REUTERS/Stringer

The raid in Katsina was the third such operation in less than a month, bringing the total of people freed from abusive conditions this month alone to about 1,000.

President Muhammadu Buhari’s government is under pressure to take urgent action to free the potentially thousands of other children who remain in similar schools across Nigeria.

Another purported Islamic school, where captives were chained to walls, some beaten so badly they needed help walking, was raided in September in neighboring Kaduna state.

Two sources at the scene told Reuters that the owner of the Mal. Niga school in Katsina metropolis and five of his staff had been arrested. Police declined to comment on the raid and blocked entrance to the grounds.

The operation, mounted by Katsina police and federal police from Abuja, freed about 500 students though not all of them had been mistreated, a police source said.

One building, which was well-kept, with clean tiles on the exterior and working plumbing, held 300 pupils who were not regularly mistreated. But about 200 captives at a site next door were regularly abused.

“The second camp is the dangerous place,” a police source said. “The children were molested there.”

The source said the most unruly students and some newcomers were placed in the second building. Students at the first school were sometimes taken to the second building for abuse.

Islamic schools, called Almajiris, are common in the mostly Muslim north of Nigeria. Muslim Rights Concern, a local organization, estimates about 10 million children attend them.

At the other raided facilities, some parents thought their children would be educated and even paid tuition. Other families sent misbehaving or difficult family members and wards to them for discipline.

Buhari, whose home state is Katsina, said in June that he planned to ban Almajiris eventually but would not do so right away.

On Tuesday, an aide said Buhari had directed police: “Go out in search of these kind of centers wherever they are and disband them.”

The centers referred to the places where people are maltreated in the name of religion, the aide said. The statement did not address Almajiris at large.