Flames were shooting up along canyon ridges as thick black smoke darkened blue skies over the small city, about 30 miles north of San Diego, known for its Legoland California amusement park that was closed Wednesday because of a power outage.
Mandatory evacuations were in progress, and more than 11,000 notices were sent to homes and businesses. Local TV broadcasts showed the blaze engulfing suburban-style homes with manicured lawns.
No injuries were reported.
State fire Capt. Mike Moehler told KCAL-TV that all of the homes were in the same neighborhood and engines were being called in from across the region to help.
The blaze was one of several wildfires that firefighters in San Diego County were battling amid hot, dry and windy conditions.
Offshore wind means some coastal communities have higher temperatures than inland and coupled with low humidities, "it's just, unfortunately, a recipe for a large fire and that's what we're seeing right now," Moehler said.
Another wildfire further north, forced the evacuation of residents in military housing at Camp Pendleton, and the closure of an elementary school on the Marine Corps base. A third fire spread from a burning vehicle on coastal Interstate 5 to roadside brush near the northwest corner of the Marine base.
Earlier, authorities reported 25 percent containment of a 2.42-square-mile fire that broke out Tuesday and forced thousands of people to flee the Rancho Bernardo area of San Diego. In Santa Barbara County, a 600-acre blaze near the city of Lompoc was 50 percent contained.
State fire officials say triple digit temperatures and the drought were setting conditions for an unusually busy firefighting season.
Marine Corps officials said military fire crews on Wednesday were fighting a blaze that started at about 9:45 a.m. at the Naval Weapons Station in Fallbrook, north of San Diego, and had spread to more than 100 acres. Residents in military housing have been ordered to evacuate. Students at Mary Fay Pendleton Elementary School, located on base, were also ordered to go home.
Flames from a burning big rig on Interstate 5 on the northern Camp Pendleton coast have spread to about 3 acres of brush. The California Highway Patrol says at least one lane in each direction remains open, but traffic is backing up. It's unclear why the truck caught fire.
Further south, blazes threatened homes in Carlsbad, and prompted the evacuation of Carrillo Elementary School in the neighboring community of San Marcos.
Evacuation orders were lifted for all of the more than 20,000 residents in and around San Diego on Tuesday night just a few hours after they were called, and all but a handful of those in 1,200 homes and businesses told to evacuate in Santa Barbara County had been allowed to return.
The 2.47-square mile blaze was 25 percent contained. It was hoped that number would increase to 50 percent by day's end, San Diego Fire-Rescue spokesman Lee Swanson said.
The Santa Barbara County blaze, 250 miles to the northwest, was 50 percent contained Wednesday morning. Firefighters also adjusted its size downward to 600 acres.
Two firefighters suffered minor injuries - one heat-related and one from smoke inhalation, state fire battalion chief Ray Cheney said. Neither blaze caused any home damage, but another hot, dry and gusty day was expected as California baked in a spring heat wave as high pressure sat over the West.
In the mountains of southwestern New Mexico, crews battling a 9-square mile wildfire are preparing for high winds this week. And in the Texas Panhandle, about 2,100 residents have started returning to their homes after wildfire burned at least 156 structures. The fire in the Fritch area was 85 percent contained Wednesday.