0540 GMT July 24, 2021
Two children could be seen leaping from the third floor of the four-story School Number 175 to escape as gunshots rang out, in a video filmed by an onlooker that was circulated by Russia's RIA News Agency.
Calling the attack a big tragedy for the whole country, Rustam Minnikhanov, the head of the wider Tatarstan region, said there was no evidence that anyone else had been involved, according to Reuters.
"We have lost seven children...four boys and three girls. And 16 people, 12 children and four adults, are in hospital," Minnikhanov told state TV.
"The terrorist has been arrested. He's a 19-year-old who was officially registered as a gun owner."
Footage posted on social media showed a young man being pinned to the ground outside the school building by a police officer.
An unconfirmed social media account belonging to the alleged shooter, which was later blocked, contained posts in which he described himself as a bloodthirsty deity and said he planned to kill a "huge number" of people before shooting himself.
It was possible the toll would still rise. Six children were in grave condition in hospital, RIA said. Earlier, Russian news agencies, citing a Health Ministry source, said 11 people had died.
"We heard the sounds of explosions at the beginning of the second lesson. All the teachers locked the children in the classrooms. The shooting was on the third floor," said one teacher, quoted by Tatar Inform, a local media outlet.
A corridor inside the school was shown strewn with debris, including smashed glass and broken doors, in unconfirmed video circulated by the Baza media outlet. Another still image showed a body on the floor of a blood-stained classroom.
President Vladimir Putin ordered the head of the national guard to draw up tighter gun regulations, the Kremlin said.
The alleged gunman was issued a permit for a Hatsan Escort PS shotgun on April 28, Alexander Khinshtein, a lawmaker in the lower house of parliament, wrote on social media.
Kazan is located around 450 miles (725km) east of Moscow.
The incident was Russia's deadliest school shooting since 2018 when a student at a college in Crimea killed 20 people before turning his gun on himself.