The Saint Petersburg metro said that an unidentified object had blown up in a train carriage.
"The Saint Petersburg prosecutor's office has begun to investigate the blast in a train carriage" at the Technological Institute metro station's platform, the prosecutors' statement said.
A source in the emergencies services told Russian news agencies that "around ten people were killed", according to preliminary information.
The station is a busy hub of the underground network in the centre of Russia's second largest city.
The Saint Petersburg metro said in a statement carried by Russian news agencies that it has closed two stations, Technological Institute and Sennaya Ploshchad -- two neighbouring stations on one line -- and is evacuating all passengers.
"Evacuation of passengers is ongoing, there are people injured," it said. "An unidentified object supposedly blew up in a (train) carriage."
President Vladimir Putin, who is holding a meeting near Saint Petersburg in his official Strelna presidential palace, offered "condolences" to those hurt in the blast.
Following the reports, the Moscow metro also announced that it is "taking additional security measures" as required by law in such situations, according to the network's official Twitter account.