Nathan Judy, a spokesman for the U.S. Forest Service, said the fire was "going up and down the hills," with rough terrain restricting firefighters' ability to contain it, the Los Angeles Times reported.
Three people were reported detained, but little information about them, including whether they had been arrested, was immediately available. A news conference was scheduled for later in the day.
The so-called Colby fire was reported about 6 a.m. near Glendora on the edge of the Angeles National Forest. Judy said when he arrived at the fire it had burned four acres. It grew quickly to 40 acres and by 8 a.m. had scorched at least 125 acres.
By 10 a.m., more than 1,700 acres had been blackened.
Judy said the fire was traveling west.
Glendora City Manager Chris Jeffers said in an email a state of emergency had been declared. The city's emergency operations center has been activated.
The Los Angles County Sheriff's Department issued mandatory evacuation orders for neighborhoods in the path of the fire. Classes were canceled at several schools in the area.
Some 550 firefighters were working to contain the blaze, which has destroyed at least two homes.
Red flag warnings for Los Angeles and Ventura counties have been posted by the National Weather Service until Friday afternoon because of extremely low humidity.