An investigation into the motives behind the incident is under way.

Officers in helmets and bulletproof vests cordoned off the area around the school, located in the central Vracar neighbourhood, shortly after 08:40 local time (07:40 GMT).

“The police sent all available patrols immediately to the spot and arrested a suspected minor – a seventh grade student who is suspected of firing several shots from his father’s gun in the direction of students and school security,” the interior ministry said in a statement.

Local media carried images of what they said was the suspect being led away from the scene by police, with his hands cuffed and his head covered by a jacket.

“I saw kids running out from the school, screaming. Parents came, they were in panic. Later I heard three shots,” one student told the Serbian state broadcaster RTS.

Milan Milosevic, the father of one of the pupils at the school, said his daughter was in the class where the gun was fired and managed to escape.

“[The boy] first shot the teacher and then he started shooting randomly,” Milosevic told broadcaster N1.

“I saw the security guard lying under the table. I saw two girls with blood on their shirts. They say he [the shooter] was quiet and a good pupil. He recently joined their class.”

Milan Nedeljkovic, mayor of the central Vracar district where the school is located, said doctors were fighting to save the teacher’s life.

Mass shootings are comparatively rare in Serbia, which has very strict gun laws, but gun ownership in the country is among the highest in Europe.

The western Balkans are awash with hundreds of thousands of illegal weapons following wars and unrest in the 1990s. In 2019, it was estimated that there are 39.1 firearms per 100 people in Serbia.