A young teenage student was in police custody on suspicion of killing a teacher and wounding four other people at his school in Barcelona on Monday.

"A teacher is dead," killed "by a bladed weapon," just after classes started at the Joan Fuster secondary school in Barcelona, a police spokesman said.

The weapon used in the attack was a crossbow,  Cataluna TV3 reported, but there was no confirmation from police.

The suspect is under 14 years old and was being held by police at the campus in northeastern Spain, as medics also rushed to the scene.

Four other people were slightly injured, the police spokesman said, adding that many details of the attack remained unclear.