At least 10 people have been killed in an explosion on the subway in St Petersburg, Russian news agencies reported.
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.
Andrei Kibitov, spokesman for the St Petersburg governor, told Russian television 10 people have been killed and 50 injured in the explosion.
In Moscow, deputy mayor Maxim Liksutov told the Interfax news agency that authorities are tightening security on the subway in the Russian capital.
Russia's National Anti-Terrorist Committee, which said "several" people were killed and injured, said an unidentified explosive device went off on a train that was travelling between two stations.
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 and a train with a mangled door nearby.
Frantic commuters reached into doors and windows, trying to see if anyone was there and shouting: "Call an ambulance!"
Mr Putin was expected to hold talks with the Belarusian president later in the day.
More to follow