At least 10 people were killed and 50 injured when an explosion tore through a train carriage in the St. Petersburg underground system on Monday, Russian authorities said.
Interfax news agency quoted an unnamed source as saying the blast was caused by a bomb filled with shrapnel.
President Vladimir Putin, who was himself in St. Petersburg for a meeting with Belarussian leader Alexander Lukashenko, said he was considering all possible causes for the blast, including terrorism. He was consulting with security services.
Ambulances and fire engines descended on the concrete-and-glass Sennaya Ploshchad metro station. A helicopter hovered overhead as crowds gathered.
Video showed injured people lying bleeding on a platform, some being treated by emergency services and fellow passengers. Others ran away from the platform amid clouds of smoke, some screaming or holding their hands to their faces.
A huge hole was blasted in the side of a carriage with metal wreckage strewn across the platform. Passengers were seen hammering at the windows of one closed carriage.
Russia has been the target of attacks by separatist Islamist Chechen militants in past years. Islamic State, which has drawn recruits from the ranks of Chechen rebels, has also threatened attacks across Russia in retaliation for Russian military intervention in Syria.
Russian airforce and special forces have been backing President Bashar al-Assad in fighting rebel groups and Islamic State fighters now being driven out of their Syrian strongholds.
St. Petersburg emergency services at first said that there had been two explosions. But a source in the emergency services later said that there had been only one but that the explosion had occurred in a tunnel between stations.
The blast occurred at 2.40 p.m., well shy of the evening rush hour.
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 on particular alert against Chechen rebels returning from Syria and wary of any attempts to resume attacks that dogged the country several years ago.
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 theater 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.