Photos and video from the scene showed the bus on the steps leading into the underground passageway.
The accident happened around 3 pm (1200 GMT) near Moscow's Slavyansky Boulevard metro station, a prestigious residential area near one of the capital's main avenues.
Four people were killed and 15 others were injured after a passenger bus drove into a busy pedestrian underpass in Moscow on Monday morning, Russian news agency reported.More news: Reliance Jio Prepaid Recharge Plans
The driver, who has been detained for questioning, has been working for the bus company for 17 years and received his license more than 30 years ago.
At least four people are said to have died after a bus came off the road and hurtled down a set of stairs leading to a metro station.
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The mayor of Moscow, Sergei Sobyanin, has expressed his condolences to the relatives of those killed in the crash and stated that the authorities are providing the necessary urgent assistance to the injured.
Ten ambulances, fire brigade officials, and three medivac helicopters were on the scene, agencies reported. "We are providing emergency aid".
Sobyanin himself arrived at the scene, telling journalists he had ordered a full inspection of the entire Moscow bus fleet.More news: White House denies report that president said Haitian immigrants 'all have AIDS'