Six people were taken to hospitals with serious injuries, public broadcaster Polish Radio’s IAR news agency reported.
Ten emergency medical teams and a rescue helicopter were deployed to the scene of the accident on the Polish capital’s northern Grota-Roweckiego Bridge.
Forty people were aboard at the time of the accident, according to province governor Konstatny Radziwiłł, who was at the scene.
State news agency PAP reported that the front of the articulated bus plunged down, landing near a carriageway below, while the rear end remained stuck on the overpass.
It added that two of the injured were children.
Photo: PAP/Paweł Supernak
Sylwester Marczak, a spokesman for Warsaw police headquarters, said the most probable cause of the accident was the bus driver fainting. He added that the driver had been sober.
The site of the accident was being examined and witnesses were being questioned. Work was underway to pump the fuel out of the gas-powered bus.
In a Twitter post, President Andrzej Duda thanked rescuers for their “selfless and efficient” efforts.