The bus plunged more than 250 meters (yards) from the road late on Friday in the remote village of Cachilaguna, near the town of Aiquile some 500 kilometres (300 miles) from La Paz, they said.
"I do know that the number of dead has passed 20," presidential spokesman Juan Ramon Quintana said.
Rescue workers were at the scene on Saturday and authorities were working to determine the cause of the crash. Injured passengers were taken to hospital in the city of Cochabamba.

Copyright AFP (Agence France-Presse), 2006