Authorities are working to extricate reported five trapped occupants in a vehicle over the side on Glendora Mountain Road.
San Dimas Search and Rescue team members report that the vehicle is possibly 300 to 400 feet over the side and that at least one occupant climbed to the road, according to California Highway Patrol broadcast traffic.
Los Angeles County Firefighters were dispatched at 8:44 p.m. and have two helicopters, three engines, one truck, one squad and an urban search and rescue unit on scene, according to county fire supervisor Ray Dong.
The crash site near mile marker 6.51, according to broadcast traffic.
The extent of injuries is unknown at this time.
No further information is available.