All metro stations now closed in St. Petersburg after blast.
Number of injured in St. Peterburg's metro blast rises to 50.
An undetonated explosive device reportedly found at a St. Petersburg subway station.
U.S. Embassy spokeswoman in Moscow reacts after St. Petersburg subway blast.
Some victims of the subway blast were evacuated by helicopter.
St Petersburg residents who have cars are offering on social networks to help one another get around town after the metro was shut down.
City health authorities tell Interfax that 25 people have been hospitalized.
Russia's General Prosecutor has declared the St. Petersburg subway blast a terrorist attack.
Reaction from the President of the Parliamentary Assembly of the Council of Europe (PACE) to the St. Petersburg subway blast.
Latest update from our news desk:
Andrei Kibitov, a spokesman for the St. Petersburg governor’s office, told state-run Rossia-24 television that "according to prelinary information, 10 people were killed" in the blast, which triggered immediate fears of terrorist attack.
Kibitov said on Twitter that around 50 people were injured and that 17 emergency response teams had been mobilized. More would be activated later, he said.