Sergei KonkovTASS | TASS | Getty Images
Emergency service vehicles at the entrance to Tekhnologichesky Institut station of the St Petersburg metro in the aftermath of an explosion.
An attack on Russia’s old imperial capital would have symbolic force for any militant group, notably Chechen secessionists and Islamic State, which is now fighting Russian forces in Syria.
Chechen militant attacks in the past have largely focused on Moscow, including an attack on an airport, a theatre and in 2010 a metro train.
Video showed injured people lying bleeding on a platform, some being treated by emergency services and fellow passengers. Others ran away from the platform amid clouds of smoke, some screaming or holding their hands to their faces.
A huge hole was blown open in the side of a carriage with metal wreckage strewn across the platform. Passengers were seen hammering at the windows of one closed carriage.
Russian TV said many had suffered lacerations from glass shards and metal, the force of the explosion maximised by the confines of the carriage and the tunnel.
“I saw a lot of smoke, a crowd making its way to the escalators, people with blood and other people’s insides on their clothes, bloody faces. Many were crying,” St Petersburg resident Leonid Chaika, who said he was at the station where the blast happened, told Reuters by phone.