At least 10 people have been killed in explosions at underground stations in St Petersburg, Russian media report.News agencies reported the blasts hit the Sennaya Ploschad and nearby Tekhnologichesky Institut stations in the centre of the city.
Images posted on social media showed a carriage in Sennaya station with its doors blown out with casualties nearby.
President Vladimir Putin said all causes, including terrorism, were being investigated.
The Interfax and RIA news agencies reported that at least 50 people had been injured. Interfax said one of the blasts may have involved a device filled with shrapnel.
President Putin was in St Petersburg earlier on Monday but is now outside the city, his spokesman Dmitry Peskov said.
"I have already spoken to the head of our special services, they are working to ascertain the cause," he said, at a meeting with Belarus President Alexander Lukashenko.
Moscow metro officials said they were introducing extra security measures as a result.
Tekhnologichesky Institut serves metro lines one and two, with the first hall opening in 1955, followed by the second in 1961.
Sennaya Ploschad - the next station along metro line two - opened two years later, in 1963.