At least 1,134 homes had been destroyed and 15 people killed by five separate blazes scattered around Southern California.
Two more people were killed as the fire spread into Mexico.
The flames dotted a 100-mile line that extended from the Mexican border north to the suburbs of Los Angeles.
A handful of other fires that hadn't hit any homes also consumed tens of thousands of acres of brush and forest lands, bringing the total burned to more than 500,000 acres or about 800 square miles.
"It's a worst-case scenario. You couldn't have written anything worse than this. You can dream up horror movies, and they wouldn't be this bad," said Gene Zimmerman, supervisor of the San Bernardino National Forest, the area in which two of the most destructive fires began last week.
A blaze in San Bernardino County called the 'Old Fire', which began near the forest on Saturday, has destroyed at least 450 homes and been blamed for the deaths of two people. It was 10% contained yesterday.
A number of the fires were believed to have been started by arsonists.
Authorities released a composite sketch on Monday night of a man suspected of starting the Old Fire.
Investigators were seeking two men in their early or mid-20s who were seen throwing flaming objects from a van along Highway 18.
A lost hunter's signal flare ignited the so-called 'Cedar Fire' near the mountain town of Julian on Saturday. The state's largest blaze at 150,000 acres, the Cedar Fire killed 11 people, authorities said. The hunter may face charges.
One of the biggest fire fights last night was unfolding in the Santa Susana Mountains that separate Los Angeles' San Fernando Valley, where 1.3 million people live, from Simi Valley in neighbouring Ventura County.
Maurice Greene, the US sprinter who won a gold medal at the 2000 Olympics, had to evacuate his 9,000-square-foot home near Simi Valley on Monday.
"We have to put it in God's hands. That's all we can do," he said as he left.
That fire, which has destroyed 13 homes since it began Saturday, was burning dangerously close to a gated community of million-dollar mansions in Los Angeles' Chatsworth section. It was only 5% contained.
Conditions were equally grim in San Diego County, where ash from three large fires fell on the beaches like snow and drivers had to turn on their headlights during the day.
More than 10,000 firefighters were battling the flames, which by yesterday had already cost the state more than $24 million.
"This will be the most expensive fire in California history, both in loss of property and the cost of fighting it," said a spokesman.
The death toll -15 people killed is the highest since the devastating Oakland Hills fire that killed 25 people and destroyed more than 3,000 homes in October 1991.
On Monday, US President George W Bush declared the region a major disaster area, opening the door to grants, loans and other aid to residents and businesses in Los Angeles, San Bernardino, San Diego and Ventura counties.