en people were killed in an explosion at a metro station in St. Petersburg on Monday, the authorities said.
The blast happened in a train in between two stations, a source in Russia's emergency services said.
"There was one blast in one site in between [stations] as the train arrived at the Technology Institute station from the Sennaya [loshchad] station," the source told Reuters.
A witness saw eight ambulances near the Sennaya Ploshchad station.
Interfax news agency quoted an unnamed source as saying one of the blasts was caused by a bomb filled with shrapnel.
President Vladimir Putin, who was in St. Petersburg for a meeting with Belarussian leader Alexander Lukashenko, said the cause of the blasts was not yet clear and efforts were on to find it out. He said he was considering all possibilities, including terrorism.
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.
A huge hole was blasted in the side of one carriage with mangled metal wreckage strewn around the platform. Passengers were seen hammering at the windows of one closed carriage.
The authorities closed all metro stations in the city.
The Moscow metro said it was taking unspecified additional security measures in case of an attack there.
Thirty-eight people were killed in 2010 when two female suicide bombers detonated bombs on packed Moscow metro trains.
Over 330 people, half of them children, were killed in 2004 when police stormed a school in southern Russia after a hostage-taking by islamist militants.
In 2002, 120 hostages were killed when police stormed a Moscow theatre to end another hostage taking.