Chevron said the blast occurred at about 2:30 p.m. PST (5:30 p.m. EST) at a hydrocracker unit in Richmond, about 10 miles northeast of San Francisco.

There were no immediate reports of serious injuries, although Chevron said at least nine people had complained of breathing difficulties after they inhaled smoke and an emergency worker suffered a minor back injury in the course of responding to the blast.

Firefighters battling the blaze appeared to be having minimal success in extinguishing the vivid orange flames, which could be seen miles away by office workers sitting in skyscrapers in downtown San Francisco.

Darrell Simmons, a worker at the Chevron plant, told a television news reporter that workers in the plant reacted calmly as the emergency unfolded.

"I was working and I seen the fire and the flames, lights going off, everbody taking emergency precautions," Simmons said. "I just took my own emergency precautions and went to my designated area, and here I am."

Firefighters were sent door-to-door in neighborhoods around the refinery warning people to stay in their homes and offices and to avoid breathing the smoke.

"We tell people to shelter in place, go inside, close all doors and windows, and turn off any air conditioning," Lou Pascalli of the Contra Costa County Health Department told Reuters.

He said that while the thick, dark smoke was acrid and could causing breathing difficulties, it did not appear to be particularly toxic.

"If you're in smoke, don't breath it," Pascalli said. "This is a high carbon, gas oil burn. It's relatively clean...there is no fallout, no downwind effect that we can ascertain at this time."

"They said there is no danger," Butt told KRON. "I'm not sure exactly what that means, but that's what they told me."

Callers to local radio and television said the blast rocked the neighborhood around the refinery.

"It was a big old explosion...it felt like an earthquake," caller Jamika Jones told KRON TV.

The fire at the Chevron unit appeared to be the second serious accident in as many months at a northern California oil refinery. In February, four workers were killed after a fireball erupted at another refinery in Contra Costa County operated by Tosco Corp

West Coast oil traders reported gasoline prices were "going through the roof" on word of the blast.

Officials said the explosion appeared to have occurred in the refinery's "isomax" unit, which processes heavy fuels into gasoline and jet fuel.