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The bus was carrying a group headed to the beach in the resort town of Rincon de Guayabitos when the predawn crash occurred, Omar Landazuri of the Nayarit civil protection authorities told AFP.
"The bus overturned and began to roll down a ravine, ejecting people along the way," Landazuri added, saying that many of the victims died from the battering they received.
More than 50 people were on board when the bus plunged 100 meters (330 feet) into the ravine, said Nayarit civil protection chief Martin Tapia. Three children and the driver were among the dead. An earlier toll had put the number of dead at 21.
Although the cause of the accident remained unclear, police suspected the driver may have been speeding or fallen asleep at the wheel.
Landazuri said the bus, owned by Express Oro Tours, had been rented by a group of tourists from northern Chihuahua state for a trip to the Pacific coast.
The crash marked the fourth serious bus accident in Mexico this year.
In the most serious recent incident, a truck smashed into a bus transporting farm workers in the eastern state of Veracruz on April 20, killing 43 people.
Around 24,000 people die from road accidents in Mexico each year, according to insurance companies -- a figure almost double the annual toll from the country's brutal drug war.
Edited By Cen Fox Post Team
