The gunman was among those killed in the incident, Styrian Police said on X shortly before 1pm, local time, posting “the perpetrator is among the dead”.
Graz Mayor Elke Kahr, who had earlier told the Austrian Press Agency (APA) the attacker was killed, called the incident at the BORG Dreierschützengasse high school a “terrible tragedy”.
Ms Kahr said that many people were taken to hospitals with injuries.
Kronen Zeitung newspaper said a suspect had reportedly been found dead in a bathroom, but that could not immediately be confirmed.
Austria’s Interior Ministry said there were several fatalities but did not say how many, while police said there were also injuries, without specifying a number.
Special forces were among large numbers of police and security services deployed to the school at 10am local time after reports of shots being heard.
At 11.30 am, police said on X that the school had been evacuated and everyone had been taken to a safe meeting point.
The situation was “secured”, they said and there was no longer believed to be any danger.
Police said several emergency services, a police helicopter and Cobra, a police tactical unit, have been deployed and they told parents to go to the ASKO Stadium, the home ground of local football club ESK Graz, to meet their children.
Gunfire and screaming could be heard on footage posted on X purporting to be from the scene, as well as an injured person being carried to a helicopter by paramedics while pictures showed armed officers at the scene.
An unnamed mother whose child survived the shooting relived the distressing moment her son called her “to say he was in school and that he was being shot and that he thought he was going to die”.
She said she has “only found out now, two hours later, that he’s still alive”.
Austrian President Alexander Van der Bellen said the country “is in mourning” as “this horror cannot be put into words”.
He said the tragedy “strikes at the heart of our country as the victims were “young people who had their whole lives ahead of them”.
He confirmed that one of them was “a teacher who accompanied them on their journey.
“There is nothing that can ease the pain felt by the parents, grandparents, siblings, and friends of the murdered victims at this moment,” he said.
Austrian Chancellor Christian Stocker called it a “national tragedy that has deeply shaken our entire country”.
Offering his condolences to the victims’ families, he said “there are no words for the pain and grief that we all – all of Austria – are feeling right now”.
The Austrian Red Cross told Sky News it had initiated its so-called ‘MANV system’, a protocol for dealing with a mass casualty incident.
All hospitals in the Styria state were alerted and had to report their available capacity.
Doctors from the state hospital (LKH) and accident hospital (UKH) were working on-site and two care centres have been set up near the school to support those affected.
The service said 158 paramedics were on site, along with 31 crisis intervention staff, a number that is expected to rise, given the large number of people affected.
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A total of 65 emergency vehicles from eight districts in Styria are in operation, it said.
Graz is Austria’s second-largest city, and has a population of around 300,000.
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