At least 10 people have been killed and 50 others were injured when an explosion ripped through a subway train in St Petersburg, the city governor's spokesman told Russian television.
President Vladimir Putin, who was visiting the city on an unrelated trip, said investigators were looking into whether the explosion was a terror attack or if there might have been some other cause.
He offered his condolences to the families of those killed.
2 explosions in St. Petersburg, #Russia metro. 10 reported dead, 30+ injured. Seems like terror, a first for St. P https://t.co/Ft92DhhKF4 pic.twitter.com/4wuKqNJhWR
— Matthew Kupfer (@Matthew_Kupfer) April 3, 2017
Russia's National Anti-Terrorism Committee said an unidentified explosive device went off on a train that was travelling between two stations.
It said security will be tightened at all critical transport facilities following the blast.
Maxim Liksutov, Moscow's deputy mayor, told Interfax that authorities were tightening security on the subway in the Russian capital.
The agency that runs the subway said several stations in the northern Russian city were closed and that an evacuation was under way.
Social media users posted photographs and video from a subway station in the city centre, showing people lying on the floor outside a train with a mangled door.
Frantic commuters reached into doors and windows, trying to see if anyone was there and shouting: "Call an ambulance!"
The explosion happened between Sennaya Ploshchad and Tekhnologichesky Institut stations.
Mr Putin was expected to hold talks with the Belarusian president later in the day.