Emergency sources say at least five people have been killed in the incident and around 15 more have been injured.
The source told Sputnik: “According to preliminary information, a bus drove into the underground passage at the metro station Slavyansky Bulvar.”
Officials are investigating how the horror crash happened but believed the brakes may have failed.
Video shows the bus driving down the stairs of the underground station.
The steps at Kutuzovsky Avenue, leading to the station, is in the west of the capital.
Officials say the incident is not believed to be linked to terror.
The driver of the bus has reportedly been detained by police.
Moscow mayor Sergei Sobyanin has expressed condolences to the relatives of the victims.
The mayor is said to already be at the scene of the crash.
Three air ambulances have left the scene.
More to follow…