At least 10 people have been killed and 50 more injured in explosions on the St Petersburg metro, Russian authorities have confirmed.
Russia's anti-terrorism committee also says it has found and deactivated a bomb at another St Petersburg subway station, the Associated Press has reported.
According to some reports there were twin blasts at Sennaya Ploschad and Tekhnologichesky Institut, caused by suspected improvised devices. However, an emergency services source told Reuters just one blast had taken place between two stations.
News agency Interfax reported the explosive device was left in a briefcase in a metro carriage.
The blast occurred at 2.40pm, and the authorities have since closed all St. Petersburg metro stations. The Moscow metro has tweeted it will be taking additional security measures in the wake of the explosion in St Petersburg and also offered help to St Petersburg metro if needed.
Russian President Vladimir Putin, who is in the city for a meeting with Belarussian leader Alexander Lukashenko, said the cause of the blasts was not clear and efforts were underway to find out.
He said he was considering all possibilities, including terrorism.
"I have already spoken to the head of our special services, they are working to ascertain the cause (of the blasts)," Putin said.
"The causes are not clear, it's too early. We will look at all possible causes, terrorism as well as common crime."
A witness told Reuters they saw eight ambulances near the Sennaya Ploshchad metro station. Three stations have been closed, local media outlet Fontanka has reported.