Preliminary reports indicate that some 10 people may have been killed and 20 injured in the blast, news agencies said citing sources close to the investigation.
The incident reportedly happened at the Sennaya station of the northwestern Russian city’s underground transit system.
The local rescue services told TASS they were responding to the report of the smoke, but could not provide any detail.
At least eight stations of the metro have been closed for passengers.
Some of the images show several bodies lying next to the damaged car, with blood apparently spilt on a bench.
RIA Novosti said evacuation of people from the affected area was underway.
The metro management said they received reports of an explosion inside the car, possibly of an improvised explosive device.
Photos taken near the Sennaya Ploshchad station entry showed firefighter vehicles involved in responding to the emergency.