Hours later, anguish and fear rose again when police found and defused a shrapnel-packed explosive device at another St. Petersburg station.
There was no immediate claim of responsibility for the attack, which came while President Vladimir Putin was visiting the city, his hometown. In the past two decades, Russian trains and planes have been frequent targets of terrorism, usually blamed on Islamic militants.
News reports said police were searching for two suspects, and Russian state television showed a photo of one suspect wearing what appeared to be a skullcap characteristic of Russia's Muslim regions.
St. Petersburg, a major tourist destination famed for its imperial palaces and lavish art museums, had been spared previous attacks.
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"From now on, I will be scared to take the subway," said Marina Ilyina, 30, who brought flowers to the station where the train stopped after the bombing. "We in St. Petersburg thought we wouldn't be touched by that."
The explosion occurred in midafternoon as the train traveled between stations one of the city's north-south lines.
The National Anti-Terrorism Committee said the death toll was 11, with another 45 people being treated for wounds in hospitals.
Amateur video broadcast by Russian TV showed people lying on the platform of the Technological Institute station, and others bleeding and weeping just after the damaged train pulled in.
"Everything was covered in smoke. There were a lot of firefighters," Maria Smirnova, a student on a train behind the stricken one, told independent TV station Dozhd.