At least eight people were killed and 20 wounded by an explosion at a college campus in the heart of the northern Nigerian city of Kano during school hours on Monday, police said.

It was not immediately clear who was behind the explosion. Bombings and attacks now happen almost daily in Nigeria's north, where militant group Boko Haram is trying to carve out an Islamist state.

The blast at Kano State School of Hygiene tore through an area just inside the main gate, where students often gather at food kiosks between classes.

Police took one suspect in for questioning, the state police commissioner, Adenrele Shinaba, told reporters. The blast was so severe that several cars parked nearby were badly damaged, he said.

Witnesses said the blast caused workers to flee the city centre of Kano, the north's largest city.

Boko Haram gained global attention in April when it abducted more than 200 girls from a rural school in north-east Nigeria.

Kano was hit by unrest this month when hundreds of youths took to the streets to protest against a decision to appoint Nigeria's former central bank governor as the country's second-highest Islamic authority.

Last month a suicide car bomb in the city killed five people on a street lined with popular bars and restaurants, in an area mostly inhabited by southern Christians.

Amnesty International estimates that more than 1,500 people were killed in north-east Nigeria in the first three months of this year.