Eleven people were killed and 45 more injured after an explosion ripped through a train on the Metro system in the Russian city of St Petersburg.
The bomb blast tore through a subway train deep under the country’s second-largest city as it travelled between the Sennaya Ploshchad and Technological Institute stations.
Anti-terror officials found another bomb at a subway station nearby which they deactivated and "made safe".
Prime Minister Dmitry Medvedev said in a Facebook post that the explosion was a “terrorist attack”.
The Investigative Committee, the country's top criminal investigation body, said it had begun a probe based on the assumption that it was terrorism but added that other possibilities were being considered.
Local media reported that police were searching for two suspects, and Russian state television showed a photo of one suspect wearing what appeared to be a skullcap.
Scores of people laid floral tributes at the station where the train stopped after the bombing.
Marina Ilyina, 30, who laid flowers at the scene, said: "From now on, I will be scared to take the subway. We in St. Petersburg thought we wouldn't be touched by that."
Alexander Malikov, 24, who brought flowers and candles to an improvised memorial outside one of the stations, said: "They won't succeed in breaking up our country. We are all citizens of one country despite various political views and religious beliefs."
Senior investigator Svetlana Petrenko told Russian media that the train driver’s decision to continue to the next station after the explosion, which occurred at about 2.30pm local time had almost certainly helped save lives as it allowed people to be rescued quickly.
It also reduced the danger to passengers who would have had to walk along the electrified tracks.
The National Anti-Terrorism Committee said the death toll was 11, with another 45 people being treated for wounds in hospitals.
Blurry video footage showed people lying on the platform of the Technological Institute station, and others bleeding and weeping just after the damaged train pulled in.
"Everything was covered in smoke. There were a lot of firefighters," Maria Smirnova, a student on a train behind the stricken one, told independent TV station Dozhd.
A female eyewitness described the scene, saying: "People were in blood, with their hair burnt out. There was smoke coming out of metro carriages.
"We were told to move to the exit because traffic was stopped. My friend was in the car next to the one that exploded.
"She said that it was shaking. When she left she saw that everyone there was mutilated."
Within two hours of the blast, authorities had found and deactivated another bomb at the busy Vosstaniya Square station, the anti-terror agency said.
That station is a major transfer point for passengers on two lines and serves the railway station to Moscow.
Russian law enforcement agencies confirmed the device was loaded with shrapnel, and the Interfax news agency said it contained up to 1 kilogram of explosives.
The entire St Petersburg subway system was shut down and evacuated but partial service resumed after about six hours.
Security was immediately tightened at all of the country's key transport hubs including the Moscow subway.
Putin, who was meeting with the president of Belarus at the Constantine Palace on the city's outskirts, offered condolences on national television.
"Law enforcement agencies and intelligence services are doing their best to establish the cause and give a full picture of what happened," Putin said.
The bombing drew widespread condemnation from politicians in other countries around the world.
President Donald Trump said it was "absolutely a terrible thing", while White House Press Secretary Sean Spicer said the US was prepared to offer assistance to Russia.
French Foreign Minister Jean-Marc Ayrault said the country would "stand by all those who suffer."
EU foreign policy chief Federica Mogherini sent a condolence message to Russian Foreign Minister Sergei Lavrov, saying she and her colleagues were following the news from St. Petersburg "with a lot of apprehension."
Russia has been the target of attacks by Chechen militants in past years and Chechen rebel leaders have frequently threatened further attacks.
At least 38 people were killed in 2010 when two female suicide bombers detonated bombs on packed Moscow metro trains.
Over 330 people, half of them children, were killed in 2004 when police stormed a school in southern Russia after a hostage taking by islamist militants. In 2002, 120 hostages were killed when police stormed a Moscow theatre to end another hostage taking.
Islamic State, which has drawn recruits from the ranks of Chechen rebels, has also threatened attacks across Russia in retaliation for Russian military intervention in Syria.
Russian air force and special forces have been backing President Bashar al-Assad in fighting rebel groups and Islamic State fighters.