The blast occurred at 14:30 in the third rail car before the subway train moved from Technological Institute station to the Sennaya Square station, various media outlets reported, according to Interfax.
A source told RIA Novosti there were two explosions - at Sennaya Square and Technological Institute stations. In turn, an Interfax source said that there was only one explosion in the St. Petersburg subway, while initial reports of the second one were not confirmed.
City police press service has earlier told UNIAN that the explosion killed at least ten people, while RIA news reported of 47 people injured.
A report by Life.ru claimed at least 12 people were killed in the attack. Eight bodies were allegedly found in the rail car, two - at the subway station, and another two in the tunnel, according to Life.ru.
The latest report by the National Anti-Terrorist Committee, eleven people were killed and 45 injured in the attack, RBC wrote.
The source told Interfax all people who were killed in the attack traveled in the affected rail car, in the immediate vicinity of the epicenter of the explosion.
The earlier reports said a supposed perpetrator, who was seeing sporting a beard and a dark hat, had reportedly left a backpack with an IED in one of the subway cars and left, according to Ren.tv.
According to a late night report by MK.ru, the man depicted in the photo turned himself in to the police claiming he had nothing to do with a terrorist act.
Первый подозреваемый ("Хаттабыч") во теракте в питерском метро, задержан. pic.twitter.com/Xn7a6zdccl— Мистер Твитлер (@Mr_Lumin) April 3, 2017
Life.ru, referring to their source in law enforcement agencies, reported that the improvised explosive device was filled with projectiles.
Read alsoExplosion hits metro in Russia's St. Petersburg, at least 10 feared dead — mediaAnother IED has reportedly been found at Vosstaniya Square subway station. It failed to go off, and a bomb squad has been deployed at the scene, Fontanka reported.
Russia's TV Channel 5 has later published on Twitter a screenshot allegedly showing another suspect, the one who supposedly planted the IED that failed to go off.
Russian President Vladimir Putin commented on the incident, saying that all versions of the blast are being considered, including a terrorist act, according to Interfax.
At the same time, the head of the Federation Council's Defense Committee has already stated he believes the explosion is a terrorist attack.
The National Antiterrorism Committee reported that the evacuation from the St. Petersburg subway had been "essentially" completed.
A group of forensic experts and detectives has been deployed in St.Petersburg by Russia’s Investigative Committee.
Interfax also said citing its source that an alleged perpetrator had been caught on CCTV cameras.
According to a later report by Interfax, their source in the law enforcement agencies said one of the versions considered is that the IED was activated by a suicide bomber, a 23-year old man, native of a Central Asian state.
UNIAN gathered the first videos from the scene uploaded online.
WARNING: Graphic content
Pictures have also been published from the scene.