An explosion in a metro station in St Petersburg has killed at least 11 people and injured at least 45 others, according to reports.
The man suspected of the attack is in his 20s and belongs to central Asia, according to BBC.
Russian President Vladimir Putin, who is in St Petersburg for a meeting with Belarussian leader Alexander Lukashenko, said the cause of the blasts was not clear and efforts were underway to find out. He said he was considering all possibilities "including terrorism".
Interfax news agency quoted an unnamed source as saying the blast, which occurred when the train was between two stations, was caused by a bomb filled with shrapnel. The blast occurred at 2:40 pm(local time) before evening rush hour.
An "unexploded device" was found at the second St Petersburg metro station, an official told AFP.
The Life News website showed pictures of blown-out train doors and injured people on the station platform.
The St Petersburg metro said evacuation of passengers is ongoing, there are people injured. "An unidentified object supposedly blew up in a (train) carriage," the statement added.
An image of the blast site at St Petersburg's metro station (Twitter)
Another agency, RIA Novosti, said there appeared to have been blasts at two metro stations but it is still not confirmed. Russian metro authorities have closed all St Petersburg metro stations. The Moscow metro said it was taking unspecified additional security measures in case of an another attack.
President Vladimir Putin later went to lay flowers at the scene of the deadly blast.
Putin, who made no comment, left a bouquet of red flowers at the entrance to the Technological Institute metro station.
Russia has been the target of attacks by Chechen militants in the past. 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.