A seven-year-old child was killed by a knife-wielding assailant who attacked a teacher and pupils at a Zagreb primary school on Friday, government ministers said.

Health Minister Irena Hrstic said the attacker stabbed five people, wounding four and killing one. The suspect later injured himself and was detained.

"Five persons have been hospitalised and their lives are not in danger," Hrstic told reporters. They included the suspected attacker.

Interior Minister Davor Bozinovic said the attacker was a 19-year-old former school student, who entered the school in the middle of the morning and wounded the teacher and children with a knife.

"He ran away from the crime site and shut himself in a nearby health centre where he tried to injure himself with the knife," Bozinovic told reporters. "The police stopped him from committing suicide."

Bozinovic said the suspect had a history of psychological problems and had tried to take his own life last year.

Zagreb Mayor Tomislav Tomašević proclaimed the Day of Mourning on Friday.

"We are horrified," Prime Minister Andrej Plenkovic said.