A wildfire in Southern California scorched more than 6,000 acres and forced the evacuation of hundreds of people from a gated community nearby, authorities said Sunday.

The flames spread quickly through a line of narrow canyons and steep hillsides covered with dense, old-growth brush, burning on both sides of Highway 101 (search) about 27 miles north of Santa Barbara.

Fire department spokesman Barry Peckham said temperatures in the area were expected to reach 90 degrees, making it easier for the fire to spread.

"There's quite a potential for extreme fire behavior," he said.

More than 300 firefighters battled the blaze, which was reported shortly before noon on Saturday, said Charlie Johnson, spokesman for the Santa Barbara County Fire Department (search).

Waves of thick, black smoke and fiery embers cut visibility on the freeway to zero, forcing authorities to divert traffic on a roughly 3-mile-long stretch of the highway in both directions. The highway was expected to remain closed for several hours.

Officials had no immediate estimate of when the fire could be contained.

About 500 people in roughly 200 homes in the community of Hollister Ranch (search) were evacuated, Johnson said. No injuries were immediately reported.

At least one structure, part of an abandoned school, was damaged by flames. The Arguello and Shell Oil refineries were threatened, but firefighters were able to protect them, spokesman Jan Purkett said.

A section of the Union Pacific Railroad railway between San Luis Obispo and Santa Barbara was closed because several trestles were damaged by the fire.