At least 11 people died and dozens were injured after a bomb exploded on a train in St Petersburg, in what authorities believe was a terror attack.
Citing law enforcement officials, the Interfax news agency 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.
Vladimir Putin is among those who have laid tributes for the victims outside Technology Institute station.
Shocking images and videos emerged, as commuters fled a subway carriage that was decimated by the blast as the train travelled between the stations Sennaya Ploschad and Technologichesky Institute at 2:30pm local time on Monday.
A huge hole was blown open in the side of a carriage with metal wreckage strewn across the platform.
Health Minister Veronika Skvortsova said 37 people were hurt.
Passengers were seen hammering at the windows of one carriage as the train came to a stop. Russian TV said many had suffered cuts from glass shards and metal.
Russia’s Prime Minister Dmitry Medvedev said the explosion was a “terrorist attack”.
News agency Interfax said the explosive device was an improvised bomb filled with shrapnel carried on to the subway in a suitcase.
Leonid Chaika, who said he was at the station where the bomb exploded, said he saw masses of injured passengers flee covered in blood.
“I saw a lot of smoke, a crowd making its way to the escalators, people with blood and other people’s insides on their clothes, bloody faces. Many were crying,” he told Reuters by phone.
Rick Macy, a 52-year-old from the US who works in St Petersburg, described what he heard while travelling in a metro carriage three stops from the blast.
He told the BBC: “What a horrific thing. You could tell there was something happening, the dispatcher had a strained voice. When I heard that, I got off the train and came off the metro. It was horrible.
“Out on the streets, where I can see from our apartment, [there are] lots of people trying to get home. We are right by the Petrogradskaya, close to the station where the bomb went off. I am grateful that we are safe. We are going to stay home tonight.”
Elsewhere in the city, Russia’s anti-terrorism committee said it has found and deactivated a bomb at another subway station.
Interfax credited a source as saying police had found CCTV that captured images of the “organiser”. Unconfirmed pictures of the person circulating on social media show a bearded man wearing a hat.
The Russian General Prosecutors Office has confirmed it believes today’s events were a coordinated terror attack.
Russia’s health minister also revealed that 10 people were killed - seven at the scene of the blast, one in an ambulance and two at hospital.
President Vladimir Putin was in St Petersburg today to meet his Belarusian counterpart, Alexander Lukashenko.
Putin said at a press conference with Lukashenko minutes after news of the explosion broke: “I have already spoken to the head of our special services, they are working to ascertain the cause.”
He has since left the city, his spokesman Dmitry Peskov said.
The entire St Petersburg metro system was evacuated and shut down by 3:40 pm local time - just over an hour after the first explosion.
British Foreign Secretary Boris Johnson tweeted his sympathies to the victims and their families, saying he was “horrified” by news of the attack.
Russia has been rocked by attacks on its public transport system before.
In December 2013, two suicide bombings on a train and bus respectively killed 34 people in Volgograd. Just two months earlier, seven people were killed when an explosive belt was detonated on a bus in the same city.
In the Russian capital of Moscow, 40 people were killed and 100 injured in an attack targeting the metro system in March 2010.
The Moscow metro said it was taking unspecified additional security measures in case of an attack there, Reuters reported.