Police in northern Nigeria on Thursday charged a cleric with operating an "illegal detention centre" after scores of young men were rescued from his Islamic school where they were tortured and chained.
The men were freed in a raid by armed police on Tuesday in the northern city of Katsina.
It was the third operation to free abused male students from boarding houses in less than a month and shone a spotlight on Islamic institutes unregulated by the authorities in the north of the country.
Salisu Hamisu, who also goes by the name Malam Niga, was arraigned before a judge in Katsina along with two assistants over their roles at the boarding school.
It had operated for 15 years and also served as an informal rehabilitation centre for drug addiction.
Katsina state police spokesperson Gambo Isa told AFP that the cleric "was charged with operating an illegal detention centre where people were kept against their will, chained and abused".
The exact number of young men held at the school was not immediately clear.
Another school in the town of Daura, 70km away, was raided by police on Monday, freeing more than 300 students who said they were chained, tortured and sexually abused.
Isa declined to give the number of inmates rescued from Malam Niga's school, but only said they were "by far fewer than those rescued in Daura".
Last week, police in the city of Kaduna raided a building to find hundreds of men and boys - some reportedly aged as young as five - held in atrocious conditions at a facility proprietors described as a religious school and rehabilitation centre.
Private Islamic schools - known locally as Almajiri schools - are widespread across mainly Muslim northern Nigeria, where poverty levels are high and government services often lacking.
High rate of drug abuse and lack of rehabilitation facilities in northern Nigeria are forcing parents to enrol their children in informal reformatory Islamic schools where they are subjected to abuse.
The authorities have estimated that there are more than nine million students enrolled at the institutions.