At least 10 people were killed and 50 others injured when an explosion ripped through a subway train in St Petersburg.
The explosion happened between Sennaya Ploshchad and Tekhnologichesky Institut stations.
Its is still unclear if the explosion was terror-related.
President Vladimir Putin, who was visiting the city on an unrelated trip on Monday, 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.
Russia’s National Anti-Terrorist Committee said an unidentified explosive device went off on a train that was traveling between two stations.
Maxim Liksutov, Moscow’s deputy mayor, told Interfax that Moscow 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 underway.
Social media users posted photographs and video from a subway station in the city center, 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!”
(With inputs from agencies)