Some background on previous attacks from our news desk:
There was no immediate claim of responsibility.
Suicide bombers have struck several times in Moscow and other Russian cities in the last two decades, with insurgents based in Chechnya or other parts of Russia's North Caucasus often blamed for or claiming reponsibility for the attacks.
In the most recent major attack of that kind, two suicide bombings on successive days in December 2013 killed more than 30 people in the southern city of Vologograd.
A bomb blast killed 27 people on a train en route from Moscow to St. Petersburg in 2009, but there have been no major attacks in St. Petersburg, Russia's second-largest city.
Kremlin pool journalist Dmitry Smirnov posts video of Russian President Vladimir Putin at his meeting with Belarusian President Alyaksandr Lukashenka commenting on today's "tragedy."
U.S. Embassy in Russia offers condolences to victims and their families of the St. Petersburg subway blast.
British Foreign Secretary Boris Johnson offers his condolences:
As does NATO chief Jens Stoltenberg:
RFE/RL's Russian Service has compiled photos from the St. Petersburg subway blast.
Fresh casualty figures from the St. Petersburg subway blast
National Anti-Terror Committee now says 9 people were killed in the blast, with more than 20 seriously injured.
Further official update:
One undetonated explosive device was located at the Vosstaniya Square subway stop.