"I appeal to you citizens of St. Petersburg and guests of our city to be alert, attentive and cautious and to behave in a responsible manner in light of events", St Petersburg Governor Georgy Poltavchenko said in an address.
The blast occurred at 2.40 p.m., well shy of the evening rush hour. Some of the victims are children, although it's not clear if they are among the dead.
The country's health minister Russia's health minister later said the fatalities included seven at the scene, one en route to hospital and two at hospital.
There were two separate explosions, one taking place at Sennaya Ploshad station and the other at the Technological Institute station, state news agency RIA Novosti reported. The president also said that terrorist attack considered as one of the possible causes of the explosion.
Soon after the incident, an unexploded device was recovered at second Saint Petersburg metro station.
The news agency is quoting law enforcement sources as saying that police are looking for a man who is believed to have planted a device that exploded in a subway vehicle on Monday, killing 10 and wounding about 40 others.
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All Metro stations were closed for passengers.
TASS posted the YouTube video below with cellphone footage showing people walking through a smoke-filled station.
There were earlier reports of two explosions.
The blasts occurred as President Vladimir Putin visited the city for a meeting and to deliver a speech.
Putin was expected to hold talks with the Belarusian president later in the day.
A bomb blast at a St Petersburg, Russia, metro station has caused an unknown number of deaths.
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Federer has played one less match, as he received a walk-over in the quarterfinal with Nick Kyrgios pulling out due to fatigue. Federer has won 19 of 22 career meetings with Wawrinka, including a semifinal win at the Australian Open in January.
"My most honest condolences to the families and friends of the victims of the explosion. This is our common pain".
Although the cause of the explosion has not been confirmed, Gabriel says it appeared to be "a perfidious attack against innocent people". Some security experts had not yet given a clean chit to ISIS and its role behind the metro blast is investigated.
If that's the case, who had reason to conduct a terror attack on Russian transportation systems?
Russian-speaking jihadists make up the largest foreign contingent fighting on behalf of the fundamentalist Sunni group in Syria, sparking fears they could spread terrorist violence if they return to Russia. The gunman shouted, "Do not forget Aleppo!" after the assassination, which was caught on photos and video and quickly spread through the internet worldwide, before being shot and killed himself.
Two suicide bombings in the Moscow subway on March 29, 2010, killed 40 people and wounded more than 100 people.
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I would say the biggest thing we'll focus on is trying to eliminate them from getting as many post touches as possible". The Tar Heels almost saw their goals slip away amid a haze of mistakes and missed free throws in the final minute.