At least 10 people have been killed and dozens more injured in an explosion between two metro stations in St Petersburg.
The blast hit the train between the underground stations of Sennaya Ploschad and Tekhnologichesky Institut stations.
Early witness reports described blasts at both stations, but it has since become clear that it was a single explosion between the two stations.
President Vladimir Putin said all possible causes of the explosion, including terrorism, were being investigated.
A spokesman for Russia’s prosecutor general has described the incident as a “terrorist act”.
The entire St Petersburg metro system has been shut down while investigators examine the scene, and extra security measures have been put in place in Moscow.
Russia has suffered a number of terror attacks in recent years, including a double suicide bombing on the Moscow metro in 2010, which killed 38 people, and a bomb on a train between Moscow and St Petersburg in 2009, which killed 27. Both attacks were claimed by Islamic militants.