SAN FRANCISCO – An explosion in a utility chamber under a downtown shopping district 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.

“It sounded like a big poof, then there was a lot of smoke,” said Tom Demartini, a construction worker who was sitting in his truck outside the store. He said he heard the explosion and saw the sidewalk rise up six or seven inches.

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 the sidewalk and shot a manhole cover 25 feet across the street, according to Fire Chief Joanne Hayes-White.

The unidentified woman was taken to a hospital with burns and was listed in critical condition. Another woman was treated 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.