At least 10 people were killed and 50 others injured on Monday when an explosion ripped through a subway train in St Petersburg, Russian media reported.
Russia's National Anti-Terrorist Committee said an unidentified explosive device went off on a train that was travelling between two stations.
President Vladimir Putin, who was visiting the city to take part in All-Russia People's Front media forum, expressed his condolences and said investigators were looking into whether the explosion was a terror attack.
However, Prime Minister Prime Minister Dmitry Medvedev said on Facebook that it was a 'terrorist attack'.
'Those who suffered during the terrorist act in the St. Petersburg metro will be provided with all the necessary assistance. All instructions to the Ministry of Health and the Ministry of Emergency Situations are given. My most sincere condolences to the families and friends of the victims of the explosion. This is a common pain,' Medvedev wrote.
Kremlin spokesman Dmitry Peskov, however, denied media speculation that Putin was due to pass by Sennaya Metro station around the time of the blast.
Soon after the blast, an improvised explosive device was defused at another St Petersburg Metro station -- Ploshchad Vosstaniya, while three trains of the Moscow Central Ring were evacuated after an unidentified object was located.
Moscow Mayor Sergey Sobyanin ordered security to be stepped up in the Moscow Metro, the Moscow Central Ring and intercity trains, the Moscow Transport service tweeted.
Social media users posted photographs and video from a subway station in the city centre, showing people lying on the floor outside a train with a mangled door.
A witness was quoted by Russia’s Life News saying, “People were bleeding, their hair burned. We were told to move to the exit, because the movement stopped. People just fled.
“My girlfriend was in the next car that exploded. She said that he began to shake. When she came out, she saw that people were mutilated.”
Seven of St Petersburg’s metro stations have been closed -- Victory Park, Electrosila, Moscow Gate, Frunzenskaya, Technological Institute, Sennaya Ploshchad, Gostiny Dvor have been shut at the entrance and exit points.
Security measures in the Delhi Metro have been put on maximum alert with commuters made to undergo a thorough frisking.
Officials said the Central Industrial Security Force, tasked to secure over 150 stations of the rapid rail network with a daily footfall of about 26 lakh people, has issued orders to exercise the ‘highest possible measure of alert’ by strengthening regular security drills and undertaking pat-down (complete) search of the passengers.
Prime Minister Narendra Modi expressed deep sadness over the loss of lives in the blasts.
‘Deeply saddened by the loss of lives in the blasts at St Petersburg metro. Heartfelt condolences to the families of the victims,’ he tweeted.