At least 11 people have been killed in a blast on the underground rail system in St Petersburg.
Dozens of people were also injured in the explosion which occurred between Sennaya Square and Technology Institute, two busy stations in the centre of the Russian city.
Citing law enforcement officials, Interfax said a suicide bomber was responsible for the blast.
The suspected bomber was a 23-year-old from central Asia who had links to radical Islamists, it added.
Following the blast, a second explosive device was found and deactivated at nearby Vosstaniya Square station.
It was made to look like a fire extinguisher, filled with shrapnel and was more than three times the size of the other explosive.
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Vladimir Putin is among those who have laid tributes for the victims outside Technology Institute station.
Earlier, the Russian president expressed his condolences and said terrorism had not been ruled out.
In a statement, St Petersburg governor Georgy Poltavchenko said the city had suffered "a terrible tragedy".
He said: "In this difficult time we all need to support those who lost their loved ones. I ask all of Petersburgers and guests of our city to be vigilant, attentive and careful."
Russian health minister Veronika Skvortsova said seven people were killed on the spot, another died in an ambulance and two others died at a hospital.
Describing the moments after the explosion, student Maria Smirnova said: "Everything was covered in smoke, there were a lot of firefighters.
"Firefighters shouted at us to run for the exit and everyone ran. Everyone was panicking."
Investigator Svetlana Petrenko said the driver of the train saved lives by continuing on to the next stop, reducing the danger to passengers who would have had to walk along electrified tracks.
All 62 stations on the St Petersburg Metro were closed after the blast.
Russia's National Anti-Terrorism Committee said security will be tightened at all critical transport facilities.
While St Petersburg's metro has not suffered attacks before, other Russian transport systems have.
In October 2013, a suicide attack on a bus in Volgograd, southwest Russia, killed six people.
Two months later, 34 died after twin bombings at Volgograd train station and on a bus in the city.