A bomb exploded on an underground train in central St Petersburg on Monday, killing at least 10 people and wounding dozens, in what Russian prosecutors are considering a terrorist attack.
Russian authorities announced a search for two people believed to have been involved in the bombing, the Interfax news agency reported.
One suspect is believed to have placed a bomb under a seat on the train, which exploded while it was travelling between stations, while the other allegedly planted a bomb that remained undetonated at a nearby station.
Authorities in Russia's second-largest city believe a homemade bomb containing shrapnel caused the blast.
Footage from the scene showed shattered windows and train doors that had been ripped open by the blast.
There were conflicting reports on how many were wounded, with state media sources saying between 20 and 50.
The prosecutor general's office condemned the blast as a terrorist attack.
All underground stations in the city have been closed. About 3 million people use the St Petersburg underground every day.
Security was increased at transport hubs across the country in the wake of the attack.
President Vladimir Putin was visiting the city, his hometown, for talks with his Belarusian counterpart, Alexander Lukashenko when the blast happened. He offered his condolences to the victims' families.
Condolences also began to arrive from leaders abroad.
US President Donald Trump denounced the attack as a "terrible thing."
"Happening all over the world. Absolutely a terrible thing,” Trump told reporters in response to a question about the attack.
"Those responsible for this appalling act must be held accountable," said a spokesman for UN Secretary-General Antonio Guterres.
The blast occurred around 2:40 pm (1240 GMT) while the train was between the central stations of Sennaya Ploshchad and Tekhnologichesky Institut, state media reported.
Security camera footage showed a suspect with a long beard and dark clothing.
Trains have been the target of attacks in the past in Russia.
In 2010, a pair of suicide bombings claimed by an Islamic separatist group on the Moscow underground killed at least 40 people.
A year earlier, that same separatist group, Caucasus Emirate, claimed it was the perpetrator of a bombing of a high-speed train en route from Moscow to St Petersburg, which killed at least 27 people.
By Peter Spinella
