A gunman has launched an attack on a school in the Russian city of Kazan that left at least nine people dead — including seven youngsters — and sent students hiding under their desks or running out of the building.

At least 21 others were admitted to hospital, six in extremely grave condition, authorities said.

The attacker, identified only as a 19-year-old, was arrested, officials said.

They gave no immediate details on a motive, but Russian media said the gunman was a former student at the school who called himself “a god” on his account on the messaging app Telegram and promised to “kill a large amount of biomass” on the morning of the shooting.

“I was in the classroom when a man with a firearm broke into our classroom and just started shooting,” said student Akhmat Khairulin. He said students hid under their desks at their teacher’s direction, though one jumped out of a window.

Attacks on schools are rare in Russia, and President Vladimir Putin reacted by ordering the head of the country’s National Guard to revise regulations on the types of weapons allowed for civilian use.

A Russian police special unit near the scene at a school after a shooting in Kazan
A Russian police special unit near the scene in Kazan (The Investigative Committee of the Russian Federation via AP)

Four boys and three girls, all eighth-graders, died, as well as a teacher and another school employee, said Rustam Minnikhanov, governor of the Tatarstan republic, where Kazan is the capital.

The teacher who died, Elvira Ignatyeva, had been an English instructor at the school for four years, the state news agency Tass reported.

Footage released by Russian media showed students dressed in black and white running out of the building. Another video depicted shattered windows, a stream of smoke coming out of one, and the sound of gunfire. Dozens of ambulances lined up at the entrance.

Russian media said while some students were able to escape, but others were trapped inside during the ordeal.

An ambulance and police trucks at a school after a shooting in Kazan, Russia
An ambulance and police trucks at the school (Roman Kruchinin/AP)

“The terrorist has been arrested, 19 years old. A firearm is registered in his name. Other accomplices haven’t been established. An investigation is under way,” Mr Minnikhanov said.

Authorities said the 21 in hospital included 18 children.

Authorities announced a day of mourning on Wednesday and cancelled all classes in Kazan schools. Authorities tightened security at all schools in the city of about 1.2 million people, 430 miles east of Moscow.

The deadliest school attack in Russia took place in 2004 in the city of Beslan, when Islamic militants took more 1,000 people hostage for several days. The siege ended in gunfire and explosions, leaving 334 dead, more than half of them children.