At least 10 people have been killed in explosions at underground stations in St Petersburg.
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 badly-mangled carriage, with a number of 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, and that a number of children were among those hurt.
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.
The entire St Petersburg underground network has now been shut down, and Moscow metro officials said they were introducing extra security measures as a result.
St Petersburg’s metro system is the 19th busiest in the world, with more than two million passengers every day.
While it has not suffered attacks before, in 2009, a bomb exploded on a high speed train travelling between Moscow and St Petersburg, killing 27 and injuring another 130.
The attack was later claimed by an Islamist group.