At least 10 people were killed in explosions in two train carriages at metro stations in St. Petersburg on Monday, Russian authorities said.
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 underway to find out. He said he was considering all possibilities including terrorism.
A Reuters witness saw eight ambulances near the Sennaya Ploshchad metro station.
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 whole 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.
Authorities closed all St. Petersburg metro stations. The Moscow metro said it was taking unspecified additional security measures in case of an attack there.
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.
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.
Putin, as prime minister, launched a 1999 campaign to crush a separatist government in the muslim southern region of Chechnya, and as president continued a hard line in suppressing rebellion.