Authorities shut down the metro system in Russia's second city for several hours as security services said they had also defused a bomb at a second metro station.
Russia's Investigative Committee said it was probing an "act of terror" but added it would look into all other possible causes of the blast.
Pictures screened on national television showed the door of a train carriage blown out, as bloodied bodies lay strewn on a station platform.
Health minister Veronika Skvortsova said the blast had killed seven people on the spot, with three more succumbing to their injuries later.
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Thirty nine people were hospitalised, including a 15-year-old girl, Skvortsova said.
"I will be afraid to take the metro now," said Maria Ilyina, 30, standing near the station, where people improvised a memorial by bringing flowers to the scene.
"Before we thought that this would not come to Saint Petersburg, now our city is under threat," she said.
"A station attendant in tears called on the men to help carry the bodies," he said.
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