City health authorities tell Interfax that 25 people have been hospitalized.
St Petersburg residents who have cars are offering on social networks to help one another get around town after the metro was shut down.
Some victims of the subway blast were evacuated by helicopter.
U.S. Embassy spokeswoman in Moscow reacts after St. Petersburg subway blast.
An undetonated explosive device reportedly found at a St. Petersburg subway station.
Number of injured in St. Peterburg's metro blast rises to 50.
All metro stations now closed in St. Petersburg after blast.
Putin offers condolences to families of victims of St. Petersburg blast.
Putin says authorities probing all possible causes for the metro blasts in St. Petersburg, including terrorism.