Four people have died after a bus came off the road and hurtled down a set of stairs leading to a metro station.

Horrifying video footage from the scene posted on social media showed a bus veering off the road and plunging down the steps of a pedestrian underpass, crushing several people beneath its wheels.

It happened on Kutuzovsky Avenue, near the Slavyansky Bulvar metro station in Moscow.

Driver Viktor Tikhonov claimed his vehicle was trapped in gear as he tried to move the bus - and he couldn't help it careering down into the underpass.

'I came to Slavyanskaya boulevard bus station, and there were 3 passengers inside,' he said.

'I was supposed to wait for 15 minutes before getting back en route.

'Another bus came to the station, I wanted to move a bit to give the other driver a bit of space.

'I took the hand break off, but couldn't do anything else as the bus drove off.

'I couldn't stop it. The bus has automatic gear, so there was nothing I could do.'

Police confirmed one of the victims was a 17-year-old girl on her way to music school and also killed was a man named as Artem Karpov, 27.

Another victim was named Yelena Mikhailova and the fourth was a woman aged around 50.

Authorities did not mention terrorism as a possible cause.

'Nine people are currently in the city's clinics... four people died on the spot,' the Moscow city hall said in a statement on the website.

Police were also giving the figure of four killed, revised down from previous statements which said the accident caused five deaths.

Footage broadcast on national television showed the bus driving onto the pavement and then down the stairs of the underpass, running over several pedestrians before coming to a stop.

Moscow police said in a statement they were looking into two possible causes - the driver losing control of the vehicle and a technical problem with the bus.

'The driver of the bus began movement and then changed his trajectory, which resulted in the bus going down into the pedestrian underpass,' the statement said.

'The driver has been detained. Police employees are working with him,' the statement said.

'According to the driver, the bus was parked and suddenly began to move by itself,' said a statement by the Russian Investigative Committee, which probes major crimes.

'Despite his attempts to stop the bus, the breaking system did not work and the bus continued moving,' it said.

Investigators said they were questioning witnesses and plan to examine vehicle documentation and order medical examination of the driver.

The accident happened around 3pm near Moscow's Slavyansky Boulevard metro station, a prestigious residential area near one of the capital's main avenues.

Sirens were blaring around the metro station, with authorities preparing to pull the bus out of the underpass, a correspondent at the scene said.

The underpass and surrounding area were cordoned off and several black body bags could be seen on the pavement, partially covered with falling snow.

The emergencies ministry said helicopters had been dispatched to the scene to help evacuate the victims.

A second video from another angle shows the bus seemingly stationary on the side of a snowy road before it appears to pull away in the direction of the metro station.

The incident occurred on one of the Russian capital's busiest roads near the Slavyansky Boulevard metro station.

Monday was an ordinary working day in Russia where Orthodox Christmas will be celebrated on January 7.

Ten ambulances, fire service personnel, and three medivac helicopters were on the scene, agencies reported.

Mayor of the capital, Sergei Sobyanin, took to Twitter to express his condolences to the families and friends of the victim.

Sobyanin also visited the scene, telling journalists he had ordered a full inspection of the entire Moscow bus fleet.

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