"The Saint Petersburg prosecutor's office has begun to investigate the blast in a train carriage" at the Technological Institute metro station's platform, a 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.
While there was no immediate indication as to what caused the blast, Russia's security services have previously said they had foiled "terrorist attacks" on Moscow's public transport system by militants, some of whom were trained by Islamic State jihadists in Syria.
And Russia's public transportation systems have been targeted by attacks in the past.
In 2013, Russia was hit by twin suicide strikes that claimed 34 lives and raised alarm over security at the Sochi Winter Olympic Games.
A bombing at the main railway station of the southern city of Volgograd killed 18 people on while a second strike hit a trolleybus and claimed 16 lives.
A suicide raid on Moscow's Domodedovo airport that was claimed by Islamic insurgents from the North Caucasus killed 37 people in January 2011.
That strike was claimed by the Caucasus Emirate movement of Islamist warlord Doku Umarov.
Russia beefed up its security over the holiday period in the wake of the attack on the Berlin Christmas market that killed 12.
Authorities placed heavy trucks at road intersections to block off areas where public festivities were taking place after the attack in the German capital that was claimed by the Islamic State group.
Russia has intervened militarily to bolster Syrian President Bashar al-Assad's forces in September 2015, turning the tables on the battlefield just as rebel forces were strengthening their hold on key areas.
Russian bombardments helped the regime retake rebel areas in the east of the northern city of Aleppo after four years of fighting.
More than 310,000 people have been killed in Syria since the conflict broke out in March 2011 with protests against Assad's rule. -- AFP