Six people died at the scene and another two people died in hospital. Most of the 12 injured spectators were airlifted to hospital.
Spectators said the truck went over a jump called "the rockpile" and sailed into the crowd. Some of the injured were crushed beneath the truck. The truck's driver was not injured but had to escape angry spectators, some of who threw rocks at the driver.
The California 200 involves off-road vehicles taking jumps and driving through other obstacles. They can reach speeds up to 100 mph during the race, which was scheduled to last most of the night The Associated Press reports.