Thousands of students of Sudan's Islamic schools, known as khalwas, are systemically chained, beaten and tortured, an investigation by BBC News Arabic has uncovered.
The revelation comes after reporter Fateh Al-Rahman Al-Hamdani, a former khalwa student himself, secretly filmed visits to 23 schools across Sudan over an 18-month period, documenting the routine abuse.
Khalwas are traditional religious schools in Sudan where children are taught to memorise the Qur'an.
The Islamic schools, which number more than 30,000 across Sudan, are typically run by sheikhs and provide students with food, drink and shelter, for free.
Al-Hamdani's footage inside the khalwas showed malnourished, unhealthy children who had been forced to sleep on the floor, even in sweltering conditions.
Many of the students, some as young as five-years-old, were shackled in heavy chains and beaten for making the slightest mistake. Sheikhs, Al-Hamdani said, were acting with impunity.
The documentary centred around the stories of Mohamed Nader and Ismail, two 14-year-old boys who were imprisoned and tortured for five days, without being given food or water, in the Al-Khulafaa Al-Rashideen khalwa.
The boys, who Al-Hamdani visited in hospital, were nearly beaten to death.
When challenged over the abuse in an interview with the BBC, Sheikh Hussein admitted children had been imprisoned, shackled and beaten in his khalwa.
He conceded that imprisoning children was wrong but claimed the practice of beating and shackling was "packed with benefits".
Sheikh Hussein went on to deny allegations that sexual abuse, including rape, had taken place in his school.
However, Mohamed Nader told the BBC the practice was widespread and that he had witnessed boys being raped by older students.
Four people, including three teachers and the head of the school, Sheikh Hussein, were charged over Mohamed Nader and Ismail's case, but all were later released on bail and are yet to face trial.
Sheikh Hussein, meanwhile, was killed in a car crash earlier this year.
The charges against him have been dropped and his brother, who has said beating children will not be tolerated under his management, has taken over the khalwa.
Sudanese authorities have long been slow to act on cases against khalwa teachers and sheikhs, but there is hope the removal of Omar Al-Bashir's government last year will leave families with a better chance of bringing those responsible to justice.
Local authorities said the ministry of religious affairs was assessing the state of the 30,000 schools across Sudan but added that it would be impossible to "solve a problem caused by 30 years of the old regime overnight".