Fires are spreading quickly in California, threatening hundreds of homes and forcing evacuations at a popular lakeside campground and a summer camp where flames temporarily trapped children.

One fire that started early Saturday afternoon is now "completely out of control".

County fire Captain Dave Zaniboni says about 90 children and 50 counsellors were trapped and had to take shelter before they were safely evacuated.

The fire is one of three in the state that grew quickly as much of California baked in heat that's broken records in Southern California.

A record that stood 131 years in Los Angeles was broken when the temperature spiked at 36°C downtown.