The blaze in Mt. Diablo State Park in Contra Costa County had burned 1,500 acres as of Monday morning, up from about 800 acres the previous day, according to the California Department of Forestry and Fire Protection. It was 10 percent contained.
Roughly 25 homes were evacuated overnight, officials said.
The blaze broke out Sunday amid temperatures near triple digits in the early afternoon.
By nightfall, it had surged to 800 acres, state fire officials said. The fire spewed a plume of smoke visible for miles and led to the evacuation of 50 to 75 homes in Clayton, a town of about 11,000 people northeast of San Francisco, alongside the park.
Firefighters are facing erratic winds, high temperatures and steep, rugged terrain. But they're confident they will get the blaze under control, Calfire spokesman David Shew said.
"We'll get it, but it will probably be a few days," Shew said. Hundreds of additional firefighters were expected to join the fight Monday.
Meanwhile, an evacuation center has been established at Clayton Community Library. Though not far from more densely populated parts of the Bay Area, the threatened homes were on sparsely populated properties dotted with animal pens and shooting ranges.
Karen Cooper, who works at a kennel in the area, said she had safely evacuated 20 dogs, but two of her horses, named Butter and Pineapple, were missing.
"It's been intense," Cooper told the Contra Costa Times.
Stay with Action News and ABC30.com for more on this developing story.