The teenage suspect has been identified by police as seventh-grade student K.K. - only his initials due to his age.

Police have said he took his father's gun before shooting his teacher and then fellow students and security guards at the Vladislav Ribnikar elementary school in the capital Belgrade on Wednesday morning.

The children who died were seven girls and one boy - all aged between 11 and 14.

The school's assistant principal called the police at 8:40am local time (7:40am UK time) while the suspect himself phoned officers two minutes later and said he had shot a number of people, officers have said.

Milan Nedeljkovic, mayor of the central Vracar district where the school is located, said the teacher is fighting for her life after being shot.

Six children who were injured are also being treated with one of them, a girl, in a life-threatening condition.

According to the law on juvenile offenders and the criminal legal protection of minors, K.K. cannot be held criminally responsible because he has not yet reached the age of 14.

His father V.K., who is 48, was ordered to be detained for up to 48 hours on suspicion of committing serious offenses against general security.

Suspects' plans 'resembled a horror movie'

Body bags have been seen being carried out of the school following the tragedy.

Veselin Milic, the chief of Belgrade police, said in a news conference that K.K. had a list of children he wanted to "liquidate" on his desk at home and had been planning the attack for a month.

Mr Milic said the suspect's plans looked "like a video game or a horror movie" and were very detailed - showing classrooms he was going to enter and which children he was going to target.

The suspect first killed a guard at the school in central Belgrade and then three students in a hallway, Mr Milic added.

The police chief said the suspect then entered a classroom - apparently choosing it simply because it was close to the entrance - and opened fire again.

The suspect was carrying four Molotov cocktails, also known as petrol bombs, in his bag during the attack.

Mr Milic said police had been told by those that know him that K.K. was a "model student, a model friend".

Footage shows the suspect's head covered as officers led him to a nearby car following his arrest.

Serbia's minister of interior Bratislav Gasic has said the boy's father is also being detained.

The minister has said the gun used in the attack was legal and the father claims that it was locked in a safe but the suspect apparent knew the code.

Mr Gasic also said that the authorities have been told the boy went to shooting ranges with his father to practice using firearms.

The government has announced three days of mourning from 5 to 7 May - while classes will resume tomorrow and start with a minute silence.

Suspect was a 'quiet' student

Officers in helmets and bulletproof vests have cordoned off the area after the first mass shooting in Serbia in 10 years.

Reports in Serbia said terrified parents arrived at the school trying to find their children.

A student who witnessed part of the shooting has said she used to be in the same class as the suspect.

She said: "He was a quiet guy, he looked nice, he had good grades, but we didn't know much about him. He was not so open with everybody. Definitely I wasn't expecting this to happen."

'My child survived this'

Speaking about what she witnessed as the shooting unfolded, she said: "I was downstairs, we had sports class downstairs, I was able to hear the shooting.

"It was non-stop, it was not like one shot at a time, it was shooting without stopping.

"I didn't know what was happening, we were receiving some messages on the phone. Some kids from seventh grade second class were not replying, so we were in real fear."

Astrid Merlini, the mother of a student who witnessed part of the shooting, said: "My child survived this, you can't imagine, she saw a man falling down, shooting, she was running away, she thought the boy with [the] gun would run after her.

"She is in shock, but she agreed to talk, but she is full of adrenaline."

'Parents were in panic'

Milan Milosevic said his daughter was in the classroom where the shooting began.

He told broadcaster N1: "She managed to escape. [The boy] ...first shot the teacher and then he started shooting randomly."

He added: "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."

A girl who attends a high school adjacent to Vladislav Ribnikar told state TV RTS: "I saw kids running out from the school, screaming. Parents came, they were in panic. Later I heard three shots."

Mass shootings are comparatively rare in Serbia, which has very strict gun laws.

But the western Balkans are awash with hundreds of thousands of illegal weapons following wars and unrest in the 1990s.

Serbian authorities have issued several amnesties for owners to hand in or register illegal guns.

In the last mass shooting in 2013, a Balkan war veteran killed 13 people in a central Serbian village.