According to preliminary information, at least 15 people were hurt.
A bus left a road in Moscow before ploughing into a subway entrance, leaving four people dead and 11 injured, Russian officials say.
Police believe the crash wasn't an intentional attack, but caused by a possible mechanical issue or the driver losing control of the vehicle.
The accident occurred in a busy area near the Slavyansky Bulvar metro station when a large number of people were in the underpass.
The driver of the bus had been detained by police, agencies said, after he lost control of the vehicle.More news: Putin thanks Trump for CIA's help in stopping terror attacks
More news: United Nations chief presses for release of arrested Reuters journalists in Myanmar
More news: Gaza Terror Rocket Landed at Israeli Kindergarten
The bus following the rout Slavic Boulevard -Skolkovo belonging to Mostransavto transportation service drove into the underground passage next to 2, Slavic Boulevard building in Moscow.
The Interfax news agency said that the investigators had confirmed that the incident had occurred as a result of a technical fault with the bus. The bus swerved off a road at high speed and rolled down steps leading to a tunnel as pedestrians hurried out of its way.
It is not clear if any of the people who died were inside the bus when it crashed. Moscow mayor Sergei Sobyanin told reporters at the scene that an inspection of the entire Moscow bus fleet had been ordered.
Monday was an ordinary working day in Moscow.