California firefighters aided by subsiding winds took the offensive on Friday against two major wildfires at opposite ends of the state, one displacing 50,000 suburban Los Angeles residents and another roaring through Sonoma county’s famed wine country.

Governor Gavin Newsom declared an emergency for both blazes, which erupted hours apart amid fierce winds this week that put emergency managers on high alert and prompted utilities to impose wide-scale power outages to curtail wildfire risks.

No injuries have been reported from either the Kincade fire in Sonoma county, about 130km north of San Francisco, or the Tick fire in the Santa Clarita Valley about 65km north of Los Angeles.

But the two blazes, the worst of several large wildfires across the state this week, have destroyed dozens of homes and other structures while prompting air-quality alerts in parts of Los Angeles and the San Francisco Bay area.

The dry, hot desert winds, gusting to 70 miles per hour (112kmph), gave way to light breezes on Thursday night, slowing the fires' advance and presenting crews with a chance to make significant headway against the flames.

The lull was expected to be short-lived.

Forecasts called for heavy winds to return late on Saturday and persist through the weekend, leaving fire crews a narrow window to tamp down the blazes in Los Angeles and Sonoma counties.

Power companies, led by the state's largest investor-owned utility, Pacific Gas and Electric Co, also were bracing for a renewal of gale-force winds.

Even as PG&E worked to restore electricity to nearly 200,000 homes and businesses switched off ahead of this week's earlier bout of severe winds, the utility announced plans for a new round of precautionary blackouts expected to leave 850,000 customers without power across 36 counties in northern and central California.

The utility cited weather data showing the expected windstorm “could be the most powerful in California in decades”.