A bus left a road in Moscow before ploughing into a subway entrance, leaving at least four people dead and 15 injured, Russian media say.
CCTV video shows people scattering as the vehicle ran down the subway’s wide steps before being brought to a halt by the tunnel’s roof.
The incident occurred near Slaviansky Boulevard station in western Moscow.
The cause of the incident was not immediately clear but there was no suggestion it was a terror attack.
Another vehicle may have hit the bus or its brakes may have failed, an unnamed source in the emergency services told Ria-Novosti news agency.
According to another unnamed source, quoted by Interfax news agency, technical failure was the likeliest cause.
A preliminary examination showed the driver of the bus was sober, it added. He has been held by police.
In July 2014, 21 people were killed when a train derailed near the same station after braking abruptly.
Monday is a normal working day in Moscow, where the Orthodox Christmas will be celebrated on 7 January.