Nine people, including seven children and a teacher, were killed in a school shooting in the Russian city of Kazan on Tuesday.
The attack by a teenage gunman took place at School No 175 in the Tatarstan capital at 9.30am.
Footage posted online showed pupils fleeing the building, with the sound of gunshots in the background.
Authorities said additional security measures were immediately put in place at all schools in Kazan, about 700 kilometres east of Moscow.
Rustam Minnikhanov, president of Tatarstan, said four boys, three girls, a teacher and another adult were killed.
“The terrorist has been arrested," Mr Minnikhanov said.
"He is 19. A firearm is registered in his name.
"Other accomplices haven’t been established. An investigation is under way.”
The Interfax news agency identified the gunman as Ilnaz Galyaviev, who was enrolled at the Tatarstan University of Management, but was expelled last month for poor academic performance.
Tatarstan health officials said 20 people were taken to hospital, including 18 children, six of whom were in intensive care.
Officials ordered a minute's silence at football matches on Tuesday and Russian Cabinet and Parliament meetings honoured those dead. Wednesday was announced as a day of mourning.
While school shootings are relatively rare in Russia, there have been several violent attacks at schools in recent years, mostly carried out by pupils.
In October 2018, a teenage gunman killed 20 people at the Kerch technical college in Crimea, the peninsula Russia annexed from Ukraine in 2014.
The attacker, 18, shot himself dead. The incident led to calls for tighter gun control in Russia.
