Andrei Kibitov, spokesman for the St Petersburg governor, told Russian television 10 people have been killed and 50 injured in the explosion.
A Russian anti-terrorism committee said it has found and deactivated a bomb at another St Petersburg subway station. The blast came as Russian President Vladimir Putin was visiting the city, his home town.
Mr Putin, speaking from Constantine Palace in St Petersburg, said investigators were looking into whether the explosion was a terror attack or if there might have been some other cause.
He offered his condolences to the families of those killed. There was no immediate claim of responsibility for the blast. The unidentified explosive device went off at 2.20pm local time on a train that was leaving the Technology Institute station and heading to the Sennaya Square station, Russia’s National Anti-Terrorism Committee said.
The St Petersburg subway immediately shut down all of its stations and the national anti-terrorism body said security measures would be tightened at all key transport facilities across Russia.
St Petersburg, Russia’s second-largest city with more than five million residents, is the country’s most popular tourist destination. The two stations that were the site of the blast are some of the subway’s busiest.
Mr Putin was in St Petersburg for talks with Belarusian President Alexander Lukashenko.
“Law enforcement agencies and intelligence services are doing their best to establish the cause and give a full picture of what happened,” Mr Putin said at the start of his talks with Mr Lukashenko.
St Petersburg governor Georgy Poltavchenko is overseeing the rescue effort.
Russian transport facilities have been the target of previous terror attacks.
Double suicide bombings in the Moscow subway in March 2010 killed 40 people and wounded more than 100 people.
Chechen rebel leader Doku Umarov claimed responsibility for that attack by two female suicide bombers, warning Russian leaders that “the war is coming to their cities”.
The high-speed Moscow-to-St Petersburg train was also bombed on November 27 2009 in an attack that left 26 dead and some 100 injured. Umarov’s group also said he ordered this attack.
President Vladimir Putin, who was visiting the city on an unrelated trip, said investigators were looking into whether the explosion was a terror attack or if there might have been some other cause.
He offered his condolences to the families of those killed.
Andrei Kibitov, spokesman for the St Petersburg governor, told Russian television 10 people have been killed and 50 injured in the explosion. In Moscow, deputy mayor Maxim Liksutov told the Interfax news agency that authorities are tightening security on the subway in the Russian capital.
At least 10 people have been killed in an explosion on the subway in St Petersburg, Russian news agencies reported. Russia’s National Anti-Terrorist Committee, which said “several” people were killed and injured, said an unidentified explosive device went off on a train that was travelling between two stations.
The agency that runs the subway said several stations in the northern Russian city were closed and that an evacuation was under way.
Russian news agency TASS quoted a police source saying at least 10 people had been killed.
There are also reports of blasts at two Metro stations. Social media users posted photographs from one subway station in the city centre, showing people lying on the floor and a train with a mangled door nearby.
Russian news agencies quoted Kremlin spokesman Dmitry Peskov saying that Russian President Vladimir Putin has been informed about the explosion. Mr Putin is visiting the city today and is expected to hold talks with the Belarusian president later. Russian news agency TAAS have reported that several people are injured following the explosion.
A local law enforcement source said: “A blast occurred at Sennaya Ploshchad metro station, several people have been injured.” An explosion at the St Petersburg subway has injured an unknown number of people, Russian state media said.