Two-year BBC News Arabic investigation uncovers horrific conditions, with boys as young as five facing violence and sexual abuse

An April evening in the suburbs of Khartoum. After months of undercover work, I had learned to time my visits to khalwas, Sudan’s Islamic schools, to coincide with evening prayers. I entered while the sheikhs (teachers) and 50-odd boys dressed in their white djellabas were busy praying. As they knelt, I heard the clanking of chains on the boys’ shackled legs. I sat down behind them and started filming, secretly.

I began investigating after allegations emerged of abuse inside some of these schools: children kept in chains, beaten and sexually abused. Khalwas have existed in Sudan for centuries. There are more than 30,000 of them across the country where children are taught to memorise the Qur’an. They are run by sheikhs who usually provide food, drink and shelter, free of charge. As a result, poor families often send their children to khalwas instead of public schools.