The blast hit a metro train in Russia's second largest city, St Petersburg.
At least 11 people were killed and dozens were injured on Monday after an explosion rocked the metro system in Russia's second largest city Saint Petersburg, authorities said.
Russia's Interfax news agency quoted an unnamed source as saying the blast was caused by a bomb filled with shrapnel.
President Vladimir Putin, who was himself in St. Petersburg for a meeting with Belarussian leader Alexander Lukashenko, said investigators were looking at all possible causes for the blast, including terrorism.
Ambulances and fire engines descended on the concrete-and-glass Sennaya Ploshchad metro station. A helicopter hovered overhead as crowds gathered.
Video showed injured people lying bleeding on a platform, some being treated by emergency services and fellow passengers. Others ran away from the platform amid clouds of smoke, some screaming or holding their hands to their faces.
A huge hole was blasted in the side of a carriage with metal wreckage strewn across the platform. Passengers were seen hammering at the windows of one closed carriage.
Russian anti-terrorism committee said one more bomb was found and deactivated at another St. Petersburg subway station.
It said it was probing an "act of terror" but added it would look into all other possible causes of the blast.
Health minister Veronika Skvortsova said the blast had killed seven people on the spot, with three more succumbed to their injuries later.
Thirty nine people were hospitalised, including a 15-year-old girl, Skvortsova said.
TRT World's Ediz Tiyansan has more details.
Scenes of confusion
The blast caused scenes of confusion, with traffic blocked on Moskovsky Prospect, a busy thoroughfare, and emergency vehicles rushing to the station.
"My mom was in the metro, I don't know what's happened to her, I can't get hold of her," one woman, Natalia, said outside the station as she was trying to make a phone call on her mobile.
The spokesman for Russia's national anti-terrorism committee (NAK), Andrei Przhezdomsky, said in televised remarks that the blast occurred at 2:40pm local time (1140 GMT) and that it was looking into its causes.
Przhezdomsky said "the blast happened in a train carriage between the stations Technological Institute and Sennaya [Square]," which are next to each other on a busy line in the city centre.
The metro network announced it was shutting down entirely after evacuating all passengers.
Russia's Investigative Committee also launched a probe into the blast.
The Moscow metro also tweeted that it was "taking additional security measures" as required by law in such situations.
Three days of mourning
Saint Petersburg announced three days of mourning in the city while Putin also offered "condolences" to those hurt in the blast and to the loved ones of those killed.
Just hours after the blast, people began laying flowers by the Sennaya Square station.
"I was shocked," said local resident Alexander Malikov. "I gathered my friends around and we came to put flowers here."
EU foreign policy chief Federica Mogherini wrote on Twitter she was following developments "together with all EU foreign ministers" gathered for a meeting in Luxembourg.
"Our thoughts are with all the people of Russia," she wrote.
Extremists have targeted Russia's public transportation systems in the past.
In 2013, Russia was hit by twin suicide strikes that claimed 34 lives and raised alarm over security at the Sochi Winter Olympic Games.
A bombing at the main railway station of the southern city of Volgograd killed 18 people while a second strike hit a trolleybus, killing 16.
A suicide raid on Moscow's Domodedovo airport killed 37 people in January 2011.