Spectators react after the monster truck rammed the stand where they were watching at El Rejon park, on the outskirts of Chihuahua. REUTERS/Eduardo Alanis/El Diario de Chihuahua
The incident late on Saturday (local time) took place at the El Rejon dam on the outskirts of the city, about 370km south of Mexico's border with the United States.
Some local media reported the death toll had risen to 13, but there was no official confirmation.
The dead included two girls aged 6 and 7 and a 12-year-old boy, according to the city government's official Facebook page. Some 46 people remained in hospital on Sunday morning after 33 were discharged.
A preliminary investigation suggested the accident was due to a "technical error," the official said, adding the driver has been detained.