The injured people reportedly included three students.

The attack occurred in an elementary school in the city.

The 19-year-old attacker, believed to be a former student, entered a first-grade classroom at the Prečko Elementary School early Friday morning and attacked the teacher and several children with a knife, American news channel CNN reported.

The teacher sustained life-changing injuries and is in critical condition, the Sveti Duh Hospital was quoted as saying by CNN.

Police said the attacker had been taken into custody.

Prime Minister Andrej Plenkovic said consoled the loss of lives in the incident and posted on X: "I express my deepest condolences to the family of the student who died at the Prečko Elementary School in Zagreb. We sympathize with all the injured and their families, as well as with the students and staff of the school."

"All the injured have been treated in hospitals and we hope for their speedy recovery. Crisis teams are on the ground, an investigation is underway, and all competent authorities will determine the circumstances of this great tragedy, which has shaken us all," he said.

The PM said: "The Government will adopt a Decision to declare a Day of Mourning in Croatia on the occasion of the tragic attack on students and staff at the Prečko Elementary School."

The country's Minister of the Interior Davor Božinović said the attack left him shaken.

"We are deeply shaken, as is the entire public, by this terrible event," the Minister was quoted as saying by Total Croatia.

Attacks in schools are rare in Croatia.