The head of Russia's National Anti-Terrorist Committee said the blast hit a train between Sennaya Ploshchad and Tekhnologichesky Institut stations.
The committee said an explosive device was later found and made safe at another station nearby.
Prime Minister Dmitry Medvedev said in a Facebook post that the explosion was "terrorist attack".
An anti-terror investigation has been opened, but other possible causes are being investigated.
What happened?
The first images posted on social media showed a train at a platform in Tekhnologichesky Institut station with a hole blown in its side, along with a number of casualties.
But the Russian National Anti-Terrorist Committee later confirmed there had been only one explosion, between the two stations, at about 14:30 local time (11:30 GMT).
Senior investigator Svetlana Petrenko told Russian media the train driver's decision to continue to the next station almost certainly helped save lives, as it allowed people to be rescued quickly.
The number of casualties given by officials has differed throughout the day. In the most recent update, Health Minister Veronika Skvortsova said 10 people had died - seven at the scene, one in an ambulance and two in hospital - and that 37 people were hurt.
What was the cause?
Andrei Przhezdomsky, the head of the National Anti-Terrorist Committee, said the explosion was caused by "an unidentified explosive device" but that the exact cause had yet to be determined.
President Vladimir Putin said all causes, especially terrorism, were being investigated. He was in St Petersburg at the time of the blast, his spokesman Dmitry Peskov said.
"I have already spoken to the head of our special services, they are working to ascertain the cause," Mr Putin said, at a meeting with Belarus President Alexander Lukashenko.
The discovery of an explosive device at another station, Ploshchad Vosstaniya, indicates the attack was deliberate, and reports in Russia say the explosion came from a briefcase left on the train.
Who might be behind the blast? Analysis by Frank Gardner, BBC Security Correspondent
The Russian authorities are sensibly being cautious before apportioning blame for Monday's metro blast. An earlier quote by the prosecutor-general that it was an act of terrorism was quickly retracted.
The FSB security service, the successors to the Soviet-era KGB, have skilled investigators, including forensic explosive experts. They will have been examining security camera footage and the explosive residue for clues, as well as the second device that was found intact and defused.
Their suspicions are likely to focus on two possible suspects, neither yet confirmed. First, an IS-inspired group enraged by recent Russian airstrikes in Syria. And second, Chechen nationalists (or even a combination of both).
Chechen militants and international jihadists do have a track record of plotting to attack Russia's transport hubs, notably in Moscow. An estimated 7,000 Russians have travelled to Syria to join extremist groups - and some have returned.
Source : BBC
