At least 10 people have died in a series of explosions on the metro system in St Petersburg.
The explosions took place on carriage trains at Sennaya Ploshchad station, as well as the Institute of Technology station in the city centre.
One of the explosives was thought to contain shrapnel.
Images on social media show heavy damage, with train doors blown off and people lying motionless on the ground.
Up to 50 people have been injured by the blasts. The entire transport network has been closed in response to the incident.
Officials say a third, unexploded bomb has been found at Ploshad Vosstaniya station and made safe.
Although no group or individual has yet claimed the attack, Islamic State supporters have reportedly been sharing images of a propaganda image which appears to show an image of the Kremlin and the hashtag 'We Will Burn Rusia' (sic).
A section of the Isis poster
New York Times journalist Rukmini Callimachi, a specialist correspondent on Isis, has shared the image, pointing out that it carries a similar font and style to a poster made in the aftermath of the London attacks.
The Russian President, Vladimir Putin, has said all possible motives for the blast are being explored, including terrorism, while emergency services operating in St Petersburg have been instructed to follow terrorist attack incident procedures.
The British Foreign Office, meanwhile, has said it is ready to help any British citizens caught up in the explosions.