At least seven school children and one teacher have been killed and many more wounded after a lone teenage gunman opened fire in a school in the Russian city of Kazan, the mayor's office says.
The attack prompted a Kremlin call for tighter gun controls.
Two children could be seen leaping from the third floor of the four-storey School Number 175 to escape as gunshots rang out, in a video filmed by an onlooker and circulated by Russia's RIA news agency.
Calling the attack a tragedy for the whole country, Rustam Minnikhanov, the head of the Tatarstan region of which Kazan is a part, said there was no evidence anyone else had been involved.
"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.
His press service later said a teacher was also killed.
"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 apparently 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 remains possible the death toll will rise, with at least six children 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.
Other footage showed emergency service vehicles parked outside the school, with people running towards the building.
Russian President Vladimir Putin had 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 politician in the lower house of parliament, wrote on social media.
Kazan is the capital of the Muslim-majority region of Tatarstan and located around 800km east of Moscow.
The incident was Russia's deadliest school shooting since 2018 when 18-year-old student Vladislav Roslyakov killed 20 people at a college in Russian-annexed Crimea before turning his gun on himself.
Australian Associated Press