As many as five gunmen dressed in camouflage clothing opened fire with automatic weapons at people in the Crocus City Hall music venue near Moscow, leaving an unknown number of people dead and injured, video footage from the scene showed.
It was not immediately clear how many people had been injured and who the gunmen were, but Russian news agencies said 50 ambulance crews had been sent to the scene.
Flames leaped into the sky and plumes of black smoke rose above the venue, video showed.
"A terrible tragedy occurred in the shopping center Crocus City today," Moscow mayor Sergei Sobyanin said. "I am sorry for the loved ones of the victims."
Sobyanin said all necessary assistance would be provided to those injured during the incident.
Russian state news agencies said police and other emergency services had arrived at the scene. Some people were still inside the concert hall.
In one unverified video posted on social media, men with automatic weapons were shown firing repeatedly at screaming civilians, including women, who were cowering below what looked like an entrance sign to Crocus City Hall.
Other video footage showed a number of people lying motionless in pools of blood outside the hall. Reuters was unable to immediately verify the footage.
Another video showed the attackers shooting at people in the concert hall.
The U.S. embassy in Russia warned earlier this month that "extremists" had imminent plans for an attack in Moscow.
It issued its warning several hours after Russia's Federal Security Service, the main successor to the Soviet-era KGB, said it had foiled an attack on a synagogue in Moscow by a cell of the militant Sunni Muslim group Islamic State.