The bus fell some 80 meters (yards) at dawn on Sunday, while headed from Lima to the area of Acobamba, lieutenant Alberto Begazo told AFP.
The bus, which belonged to the company Antezana, was traveling through a curvy ravine area in the Andes.
Fifteen people were rescued, some of them in serious condition, and taken to the city of Huancayo where they were hospitalized, said Begazo, who spoke from the Acoria district near the crash site, some 440 kilometers (275 miles) southeast of Lima.
One injured person told regional police that the bus driver was traveling at excessive speed on a narrow and windy road, RPP radio reported.
Accidents are frequent in Peru where poor road conditions are compounded by routes without asphalt that wind their way over the Andes.
Inexperienced drivers and mechanical bus failure contribute as well.
According to a report by the government’s National Institute of Statistics and Information, 781 people died in Peruvian traffic accidents in the first quarter of 2013.