Another 18 people were injured, according to a provisional toll given by a highway police official in the city of Arequipa, located 1,000 kilometers (620 miles) south of the capital Lima.
Police warned the toll may rise as more bodies could be trapped under the wreckage of the vehicle, which rescue personnel has so far failed to move.
The cause of the accident was not immediately known, police said.
Most of those aboard the bus were mine workers headed from Orcopampa district to Arequipa, where they planned to take part in religious festivities leading up to Easter on Sunday.
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