The accident happened in Mestre, a suburb of Venice that is connected to the city by a bridge. The bus broke through a barrier and plunged about 10 meters (33 feet) onto the tracks, according to local media reports.
The bus was carrying 39 passengers and two drivers, most of them from Ukraine, according to the Ukrainian foreign ministry. The survivors were taken to a nearby hospital with various injuries.
The cause of the crash is still under investigation, but some witnesses said the bus was speeding and swerving before it fell off the overpass. The local authorities have declared a state of mourning for the victims and expressed their condolences to their families and friends.
This is one of the worst bus accidents in Italy in recent years. In 2017, 16 people died when a bus carrying Hungarian students crashed into a highway barrier and burst into flames near Verona. In 2013, 38 people died when a bus plunged off a viaduct in southern Italy.