At least nine people, including seven children, were killed after a gunman shot randomly in a school in Kazan, capital city of Russia's Republic of Tatarstan on Tuesday, Tatarstan's leader Rustam Minnikhanov said.

Four boys, three girls, one teacher and one school staffer lost their lives in the armed attack at the No. 175 school, Minnikhanov said.

A total of 21 people have been hospitalized, including 18 children and three adults. Eight of them remain in serious condition.

A 19-year-old suspect was arrested, according to Minnikhanov, who said no other accomplices have been identified. It was not immediately clear what the motive for the attack was. 

RIA news agency had earlier cited an emergency services source as saying that at least 11 people were killed. The initial report said another suspect was in a standoff with police inside the school, but officials later confirmed that the teenager is thought to have acted alone.

Russian President Vladimir Putin has ordered the urgent tightening of civilian weapons control following the deadly school shooting, Kremlin spokesperson Dmitry Peskov said on Tuesday.

Putin expressed deep condolences to the relatives of the children killed in the armed attack, Peskov said.

The republic declared a day of mourning on Wednesday.

(With input from Xinhua News Agency.)