SAN FRANCISCO -- A fiery explosion from an underground utility chamber in the downtown shopping district Friday critically burned a woman, sent a manhole cover flying and shattered windows at a Polo Ralph Lauren clothing store, authorities said.

The cause was still under investigation hours later, but authorities ruled out terrorism.

Sprinklers doused a flaming awning outside the store as panicked workers were evacuated from skyscrapers, stores and buildings around the Crocker Galleria area, said fire spokesman Peter Howes.

"All of a sudden there was a big, big, big boom and the lights flickered off," said Ellen, who joined other workers in a smoky stairwell as she made her way to the street from the 28th floor above the Ralph Lauren store. She wouldn't give her last name, but said she worked for Charles Schwab.

"I thought it was a bomb," she said. "It was very scary."

The blast came from a vault that houses four Pacific Gas & Electric Co. transformers fed by a 34,000-volt electrical cable, said company spokesman Paul Moreno. The cause could not immediately be determined because a bomb squad was investigating, but Moreno said there have been vault explosions in the past.

No utility workers were present when the explosion buckled a sidewalk and shot a manhole cover 25 feet across the street, according to Fire Chief Joanne Hayes-White.

Tom Demartini, a construction worker who was sitting in his truck outside the store, said he heard the explosion and saw the sidewalk rise up six or seven inches.

"It sounded like a big poof, then there was a lot of smoke," Demartini said. "One woman looked like she was badly burned."

The unidentified woman was taken to the Bothin Burn Center at St. Francis Memorial Hospital where her condition was listed as critical, said spokeswoman Linda Gillespie.

"At the request of her husband, no information is to be given out, except for a one-word description," Gillespie said.

Another woman was treated at the scene for smoke inhalation, authorities said.

Scores of police officers and firefighters closed off the area and shut down several blocks as hundreds of displaced office workers milled about the edge of the city's financial district.

San Francisco Fire Department spokesman Pete Howes said people are sheltering in place on 29 of the 37 floors at the Crocker Galleria, near the site of the explosion.

Howes said people on the lower floors of the Crocker Galleria were evacuated after the 9:54 a.m. explosion rocked the area, but people on the upper floors were told to stay in place.

Howes said authorities aren't allowing people in buildings near the explosion site to exit onto Kearny Street, but they're being allowed to leave if their buildings have alternate rear exits.

Workers said the blast shook nearby buildings and some thought it was an earthquake. One woman said the elevator in her building stopped.

Nancy Murray, a spokeswoman for Polo Ralph Lauren Corp. in New York, said the blast came as employees prepared to open the store, but no one inside was injured. She did not know the extent of damage.

PG&E said initial reports that four customers lost power were inaccurate.