Eleven people have died and 45 people are injured after after an explosion tore through a metro train carriage in St Petersburg.
Russian news media reported police were searching for a man recorded on surveillance cameras who was thought to have been involved in the attack, which coincided with a visit to the city by President Vladimir Putin.
A grainy photograph published by the Fontanka news outlet showed a middle-aged man with beard and black hat.
More than 45 were injured in the explosion between Sennaya Ploshchad and Tekhnologichesky Institut stations.
Video from the scene 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 blown open in the side of a carriage with metal wreckage strewn across the platform. Passengers were seen hammering at the windows of one closed carriage.
Senior investigator Svetlana Petrenko told Russian media the train driver's decision to continue to the next station almost certainly helped save lives, as it allowed people to be rescued quickly.
Health Minister Veronika Skvortsova said 10 people had died - seven at the scene, one in an ambulance and two in hospital. The death toll was later raised to 11.
Russian TV said many had suffered cuts from glass shards and metal, the force of the explosion maximised by the confines of the carriage and the tunnel.
Authorities in Saint Petersburg have declared three days of mourning for the victims.
President Vladimir Putin, who was in the city when the blast occurred, visited the scene on Monday evening and laid flowers at a makeshift shrine.
Prime Minister Dmitry Medvedev said in a Facebook post that the explosion was a "terrorist attack".
Russia's National Anti-Terrorist Committee said an explosive device was later found and made safe at another station nearby. Interfax news agency cited unnamed sources as saying the bomb, packed with shrapnel, may have been hidden in a train carriage inside a briefcase.
The investigative committee, a state body which investigates major crimes, opened a criminal case on charges of terrorism but said it would consider all other possible causes.
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 in the past of numerous bomb attacks, frequently targeting public transport.
Most were blamed on Islamist rebels from Russia's North Caucasus region. The rebellion there has been largely crushed, but security experts say Russia's military intervention in Syria has made Russia a potential target for Islamic State attacks.
Russia has been on particular alert against Chechen rebels returning from Syria, where they have fought alongside Islamic State.
The blast raised security fears beyond Russian frontiers. France, which has itself suffered a series of attacks, announced additional security measures in Paris.
At least 38 people were killed in 2010 when two female suicide bombers detonated bombs on packed Moscow metro trains.
More than 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.
Mr 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.
- Reuters / BBC