The bus was en route to Goalpara from Dhubri with 35 passengers when the mishap occurred beside the National Highway-37 at North Salmara part-1 here, the sources said.
The deceased have been identified as Rehana Begum (32), her daughter Mehjebin (3), residents of Malegarh area here; Sujit Das (22), resident of Bilasipara in Dhubri; and two residents of Boitamari area here - Sorobinda Ray (42) and Gofuruddin Sekh (53).
"Soon after overtaking a car, our bus lost control and rammed into the railing of the roadside bridge and fell into a gorge," said Sujit Mandal, a resident of Bahalpur in Dhubri who was among the injured.
The locals rescued the injured and rushed them to hospitals by private vehicles before the arrival of ambulance and police at the site, according to the passengers.