Russian news media reported that 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. Interfax news agency cited unnamed sources as saying the bomb, packed with shrapnel, may have been hidden in a train carriage inside a briefcase.
Russia's National Anti-Terrorist Committee said an explosive device had been found at a different metro station, hidden under a fire extinguisher, but had been made safe.
The Investigative Committee, a state body which investigates major crimes, opened a criminal case on charges of terrorism.
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.
Soon after the blast happened at 2:40 p.m., ambulances and fire engines descended on the concrete-and-glass Sennaya Ploshchad metro station as a helicopter hovered overhead.
"I appeal to you citizens of St. Petersburg and guests of our city to be alert, attentive and cautious and to behave in a responsible matter in light of events," St Petersburg Governor Georgy Poltavchenko said in an address.
The blast raised security fears beyond Russian frontiers. France, which has itself suffered a series of attacks, announced additional security measures in Paris.
Video from the scene of Monday's blast 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.
“I saw a lot of smoke, a crowd making its way to the escalators, people with blood and other people's insides on their clothes, bloody faces,,” St Petersburg resident Leonid Chaika, who said he was at the station where the blast happened, told Reuters by phone. "Many were crying."
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.
Russian TV said many had suffered lacerations from glass shards and metal, the force of the explosion maximized by the confines of the carriage and the tunnel.
ALL STATIONS CLOSED
Officials said earlier on Monday that the death toll from the explosion was 9, but the health minister later revised that upwards to 10 dead.
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, where they have fought alongside Islamic State, 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.