A fast-moving brush fire near Yosemite National Park has exploded in size into one of California's largest wildfires of the year, prompting evacuation orders for thousands of people and shutting off power to more than 2,000 homes and businesses.

The Oak Fire started on Friday afternoon south-west of the park near the town of Midpines in Mariposa County and by Saturday had grown to nearly 48 square kilometres, according to the California Department of Forestry and Fire Protection, or Cal Fire.

It erupted as firefighters made progress against an earlier blaze that burned to the edge of a grove of giant sequoias in the southernmost part of Yosemite park.

Evacuation orders were put in effect on Saturday for over 6,000 people living across a several-mile span in the sparsely populated, rural area, said Daniel Patterson, a spokesman for the Sierra National Forest.

Governor Gavin Newsom proclaimed a state of emergency for Mariposa County on Saturday due to the effects of the Oak Fire.

More than 400 firefighters were battling the blaze, along with helicopters, other aircraft and bulldozers, facing tough conditions that included hot weather, low humidity and bone-dry vegetation caused by the worst drought in decades, Mr Patterson said.

"Explosive fire behaviour is challenging firefighters," Cal Fire said in a statement on Saturday that described the Oak Fire's activity as "extreme with frequent runs, spot fires and group torching".

By Saturday morning, the fire had destroyed 10 residential and commercial structures, damaged five others and was threatening 2,000 more structures, Cal Fire said.

The blaze prompted numerous road closures, including a shutdown of Highway 140 between Carstens Road and Allred Road — blocking one of the main routes into Yosemite.

'Exceptional fire conditions'

California has experienced increasingly larger and deadlier wildfires in recent years as climate change has made the West much warmer and drier over the past 30 years.

Scientists have said weather will continue to be more extreme and wildfires more frequent, destructive and unpredictable.

"The fire is moving quickly. This fire was throwing embers out in front of itself for up to 2 miles yesterday," Mr Patterson said.

The cause of the fire was under investigation.

Pacific Gas & Electric said on its website that more than 2,600 homes and businesses in the area had lost power as of Friday afternoon and there was no indication when it would be restored.

"PG&E is unable to access the affected equipment," the utility said.

A shoeless older man attempting to flee the blaze on Friday crashed his sedan into a ditch in a closed area and was helped by firefighters.

He was safely driven from the area and did not appear to suffer any injuries.

Several other residents stayed in their homes on Friday night as the fire burned nearby.

Washburn Fire

Meanwhile, firefighters have made significant progress against a wildfire that began in Yosemite National Park and burned into the Sierra National Forest.

The Washburn Fire was 79 per cent contained after burning about 19.4 square kilometres of forest.

It was one of the largest fires of the year in California, along with the Lost Lake Fire in Riverside County that was fully contained in June at 23 square kilometres.

The fire broke out July 7 and forced the closure of the southern entrance to Yosemite and evacuation of the community of Wawona as it burned on the edge of Mariposa Grove, home to hundreds of giant sequoias, the world's largest trees by volume.

Wawona Road is tentatively set to reopen on Saturday, according to the park website.

AP