There was only one blast in St Petersburg's metro system which happened in a train in between two stations, and has killed at least 10 people, a source in Russia's emergency services says.
"There was one blast in one site in between (stations) as the train arrived at the Technology Institute station from Sennaya (Ploshchad) station," the source told Reuters, following Monday's blast.
Russian media reported earlier that there were two blasts.
Russian President Vladimir Putin, who is in the Baltic port city for a meeting with Belarussian leader Alexander Lukashenko, said the cause of Monday's blast was not clear and efforts were underway to find out.
He said he was considering all possibilities including terrorism.
Andrey Kibitov, the head of the city governor's press service said on Twitter 50 people had been injured by the blast.
Seventeen ambulances were currently treating the injured, Kibitov said.
All the city's metro stations had been closed, Interfax earlier reported.
Witnesses saw eight ambulances near the Sennaya Ploshchad metro station. Three stations have been closed, local media outlet Fontanka reported.
Video showed injured people lying bleeding on a platform, some being treated by emergency services. Others ran away from the platform amid clouds of smoke.
Russia has been the target of attacks by Chechen militants in past years. Chechen rebel leaders have frequently threatened further attacks.
At least 38 people were killed in 2010 when two female suicide bombers detonated bombs on packed Moscow metro trains.
