The bomb blast tore through a subway train in Russia’s second-largest city.
Pictures from the scene at the Sennaya Ploshchad station show people lying on the platform, and a damaged train carriage.
The blast happened between two stations.
Authorities say the driver made the right decision in not stopping the train, instead taking it to the next station so that passengers could evacuate and the injured could be helped.
Alexander Polyakov, from Russia’s National Anti-Terrorist Committee, says hours after the blast, police found an unexploded device at another metro station.
“As a result of steps taken by law enforcement bodies, a self-made explosive device was discovered and made safe at Ploshchad Vosstania metro station. Special services and law enforcement bodies continue undertaking measures to prevent terrorist threats.”
A hole was blasted through the side of a carriage with metal wreckage strewn across the platform.
Passengers were seen jumping out of the train’s shattered windows to escape the scene.
Many had suffered lacerations from glass shards and metal, with the force of the explosion maximised by the confines of the carriage and the tunnel.
One witness describes the moments that followed the attack.
“At that moment there was a huge bang. It was deafening. And I was sitting next to a metal railing and I think it saved my life. Of course everybody fell to the right. Not down to the floor, but on seats.”
The Interfax news agency says Russian police suspect the explosion was caused by a suicide bomber.
The agency says authorities have identified the suspected attacker as a 23 year-old national of an ex-Soviet Central Asian nation.
Russian President Vladimir Putin, who was in the city for a meeting, says it’s too early to tell what the cause of the incident is.
“I would like to express my sincerest condolences to relatives of the killed and wounded. I have already spoken with the chiefs of special services: FSB director, law enforcement bodies and special services. We will do everything to identify the reasons for what happened, give a full evaluation of what happened, and city and federal authorities will do everything possible to support the families of our killed and injured citizens.”
The explosion is the latest in a number of attacks targeting Russia in recent years.
Some of the deadliest include an attack in October 2015, in which militants from a local affiliate of Islamic State downed a Russian airliner en route to St. Petersburg over Egypt, killing all 224 people on board.
In October 2014 a suicide bomber wearing a police uniform blew himself up in Grozny, the capital of Chechnya, killing five policemen and wounding 12 others.