Four people are dead after a city bus ploughed into a pedestrian underpass in western Moscow on Christmas Day.

It was not immediately clear what caused the crash. Police said they were looking whether the driver lost control or if the bus malfunctioned. Terrorism has been ruled out.

"Nine people are in the city's clinics. Four people died on the spot," the Moscow city hall said on its website.

Police also said four were killed, revised down from previous statements that said the crash caused five deaths.

Advertisement

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

Russian authorities said the bus careened off a road and onto steps leading into an underground passageway in Moscow.

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

"The driver has been detained. Police employees are working with him."

The Russian Investigative Committee, which investigates major crimes, said the driver told them the brakes did not work.

"According to the driver, the bus was parked and suddenly began to move by itself. Despite his attempts to stop the bus, the braking system did not work and the bus continued moving," it said.

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

The crash happened about 3pm local time 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, as authorities prepared to pull the bus out of the underpass, an AFP 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.

Body bags lie in the street after a bus crash that killed four people. Photo / AP
Body bags lie in the street after a bus crash that killed four people. Photo / AP

The Emergencies Ministry said two helicopters had been dispatched to the scene to help evacuate the victims.

"There are dead and wounded," tweeted Moscow mayor Sergey Sobyanin. "We are providing emergency aid," he said, offering condolences to families of the victims.

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

The Russian Orthodox Church celebrates Christmas on January 7.