Nigerian police rescued more than 300 people, mostly children, being held in chains in a building in the northern city of Kaduna, a police spokesman said on Friday.
Many of the children had metal chains around their ankles, police said. The spokesman said the building housed an Islamic school and seven people have been arrested.
"The state government is currently providing food to the children who are between the ages of five and above," said Yakubu Sabo, the Kaduna police spokesman.
"We have identified two of the children to have come from Burkina Faso, while most of them were brought by their parents from across mostly northern Nigerian states," he added.
He said those arrested were teachers at the school.
Islamic schools, known as Almajiris, are common across the mostly Muslim north of Nigeria - a country which is roughly evenly split between followers of Christianity and Islam.