More than 20 thrill-seekers were trapped for nearly three hours on a high-speed roller coaster ride after a car was derailed by a tree branch.
Four people were injured on the Ninja ride at the Magic Mountain amusement park in Valencia, north of Los Angeles.
The ride was in a "precarious position" as the passengers were suspended about 12 metres above ground, said Michael Pittman, of Los Angeles County Fire Department.
The 22 people on the ride were safely evacuated around 8.30pm, Magic Mountain spokeswoman Sue Carpenter said. Two riders were taken to hospital as a precaution, but their injuries were minor.
Fire Inspector Fred Flores said the branch was lying across the tracks, derailing the first car and forcing the shutdown of the ride at about 6pm.
A specialised urban search-and-rescue team was sent to the park. Firefighters positioned ladders next to the roller coaster amid thick foliage.
Stranded passengers could be seen talking to the firefighters, who were next to the car working to free them.
Magic Mountain bills the Ninja ride as "The Black Belt of Roller Coasters", racing along a twisting track at 88km/h.
"Your whole body will swing out to the sides as you take winding snake-like turns at nearly 4G," the park says on its website.
The ride will remain closed while inspectors survey the track and the area, Carpenter said.
"The safety of our guests and employees is our No1 priority," she added.