At least 10 people died and 50 others were injured when a powerful explosion ripped through a subway train in the Russian city of St. Petersburg on Monday evening, official said.
The blast came as Russian President Vladimir Putin was visiting the city, his hometown.
Witnesses on the subway said the blast spread panic among passengers, who ran toward the exits. Russian news agency TASS reported that an unidentified device exploded in a train car.
There was no immediate claim of responsibility for the blast.
В Петербурге на станции "Технологический институт" произошёл взрыв в вагоне метро. pic.twitter.com/vx3PoLbCgJ— Лентач (@the_lentach) April 3, 2017
Within two hours, Russia authorities found and deactivated another bomb at a separate busy St. Petersburg subway station, Vosstaniya Square by the Moscow railway station, Russia's National Anti-Terrorist Committee said.
Putin, who was in St. Petersburg for a meeting with Belarussian leader Alexander Lukashenko, said the cause of the blasts was not yet clear and efforts were underway to find out.
Putin, speaking from Constantine Palace in St. Petersburg,said investigators were looking into whether the explosion on the train was a terror attack or if there might have been some other cause. He offered his condolences to the families of those killed.
St Petersburg is the country's second largest city after Moscow.
The St. Petersburg subway immediately shut down all of its stations and the national anti-terrorism body said security measures would be tightened all key transport facilities across Russia. Maxim Liksutov, Moscow's deputy mayor, said that included tightening security on the subway in the Russian capital.
Some children have been injured in the explosion, a law enforcement source told Sputnik. "There are children among the victims who were most likely on spring break. However, their number and names are unknown at the moment," the source added.
The unidentified explosive device went off at 2:20 p.m. on a train that was leaving the Technology Institute station and heading to the Sennaya Square station, authorities said.
Social media users posted photographs from one subway station in the city center, showing people lying on the floor and a train with a mangled door nearby.
Meanwhile, a spokesman for the city’s governor Andrei Kibitov said on Monday that 50 people have been injured in the incident.
"As of this minute, about 50 people were injured. Medics are working with them. There are currently 17 ambulance teams and their number will be increasing," the spokesman said.
Putin was in St. Petersburg on Monday for talks with Belarusian President Alexander Lukashenko, and went ahead with the talks after appearing on Russian television to speak about the attack.
"Law enforcement agencies and intelligence services are doing their best to establish the cause and give a full picture of what happened," Putin said.
St. Petersburg governor Georgy Poltavchenko was overseeing the rescue effort.
Russian transport facilities have been the target of previous terror attacks.
Two suicide bombings in the Moscow subway on March 29, 2010, killed 40 people and wounded more than 100 people. Chechen rebel leader Doku Umarov claimed responsibility for that attack by two female suicide bombers, warning Russian leaders that "the war is coming to their cities."
The high-speed Moscow-to-St. Petersburg train was also bombed on Nov. 27, 2009, in an attack that left 26 dead and some 100 injured. Umarov's group also said he ordered this attack.
Russian airports have also been hit by attacks. On Jan. 24, 2011, a suicide bomber hit Moscow's Domodedovo Airport, killing 37 people and wounding 180. The same airport in August 2004 saw Islamic suicide bombers board two airplanes and bring them down, killing a total of 90 people.