The fire burning near Highway 39 at East Fork Road, on the north end of the San Gabriel Reservoir, was first reported about 2:30 p.m. and had charred more than 700 acres by 5 p.m. By 9:30 p.m., it was up to 3,600 acres and 5 percent contained.
About 50 residents and 1,000 campers were evacuated from three campgrounds, including Camp Williams and the River Community rehab treatment center, officials said.
Officials said power lines were threatened. At least one power line fell and another was seen on fire.
Two-hundred firefighters were at the scene. Air units were called in to help fight the fire, including a DC-10 capable of dropping thousands of gallons of retardant.
Highway 39 was shut down from Crystal Lake Road down to the 210 Freeway, officials said. It was to remain closed through the Labor Day holiday weekend.
The cause of the fire, named the Williams Fire, was not known.
The area had not burned since 2003.
No injuries were reported.