More updates from the authorities on the casualties:
St. Petersburg Governor Georgy Poltavchenko told journalists that 10 people were killed and "around 20" were injured.
Russia's National Anti-Terrorism Committee (NAK) put the "preliminary" death toll at nine, according to the Interfax news agency.
A spokesman for Russia's Prosecutor-General's Office, Aleksandr Kurennoi, called the explosion a "terrorist act" but later said it was too early to make any definitive statement about the cause.
Poltavchenko's spokesman had said on Twitter earlier than about 50 people were injured.
Many of those injured suffered shrapnel wounds, according to news reports. An unidentified source was cited by the Interfax news agency as saying that the explosion involved a bomb packed with shrapnel.
Surveillance cameras capture images of supposed organizer of St. Petersburg subway blast, according to Interfax, citing an unidentified source.
More condolences coming in, this time from Federica Mogherina, high representative of the EU for foreign affairs and security policy.
Russian Prime Minister Dmitry Medvedev offers a messages of sympathy on Facebook:
"All necessary help will be given to those who suffered during the terrorist attack in Saint Petersburg. All the orders have been given to the Health Ministry and Emergency Ministry. My most sincere condolences to the relatives and close ones of the victims of the explosion. It is painful for all."
On the second device:
Russia's National Anti-Terrorism Committee spokesman Andrei Przhezdomsky told state television that an undetonated “homemade bomb” was later found at another central subway station, Ploshchad Vosstaniya, and “disarmed in time."
RBK reports the death toll has risen to 11, citing a report from Rosbalt that two people rushed to hospital have died.
Location of the blast:
