Here are the 10 latest developments in this story:
Quoting the National Anti-terrorism Committee, Russia Today reported that the explosion took place as the train was travelling between the stations Tekhnologichesky Institut and Sennaya Ploshchad.
The police also suspect that the explosion was caused by an Improvised Explosive Device or IED, which was hidden in a suitcase.
Another explosive device had been found at a different metro station, hidden under a fire extinguisher, but had been defused, said Russia's National Anti-Terrorist Committee.
Russian news media reported that police were searching for a man recorded on surveillance cameras who was thought to have been involved in the attack. The middle-aged, bearded suspect is seen wearing a black hat, said Fontanka news outlet.
All subway stations of St Petersburg were closed after the explosion. Commuters were evacuated minutes after the blasts were reported.
Video footage showed graphic images of smashed train doors and injured people on the platform. A huge hole has been blasted on the side of one carriage, mangled metal wreckage is seen strewn around the platform.
Passengers were seen hammering at the windows of a closed carriage.
With concerns of a potential follow-up attack, security has been tightened across St. Petersburg.
The Moscow metro said it was taking additional security measures in view of a possible attack there.
Chechen militants have carried out attacks in Russia earlier. In 2010, at least 38 people died when two women suicide bombers detonated bombs on packed Moscow metro trains.
"A Mistake": Zelensky Brushes Off Biden's Putin Mix-Up Watch: How Zelenskyy Reacted When Biden Called Him "President Putin" "No Good Reason" To Talk To Putin, Says US President Biden Barack Obama Wants Joe Biden To Pull Out Of US Presidential Race: Report World's Largest Isolated Tribe Makes Rare Appearance In New Footage 32 Dead In Bangladesh Unrest, Protesters Set Fire To State TV Headquarters Comedy Legend Bob Newhart Dead At 94: Publicist Israeli Strike Kills Field Commander In Elite Hezbollah Unit: Report Delhi-San Francisco Air India Flight Diverted To Russia After Engine Glitch Track Latest News Live on NDTV.com and get news updates from India and around the world.