San Diego Gas and Electric, which supplies power to much of the region, said that the exact cause was unknown, but that the system may have been overwhelmed by a heat wave. Officials said the blackout was not the result of terrorist activity.

All of the company’s 1.4 millions consumers were without power on Thursday, as the failures, centered in San Diego, stretched as far north as Orange County, south into Mexico, and east into Arizona. On its Web site, San Diego Gas and Electric said the blackout could continue into Friday and urged residents to exercise care in driving and stay home at night.

San Diego residents described a chaotic scene after the power went out around 4 p.m. Police officers were sent to busy intersections to direct traffic. Without the Internet, lights or any idea when power might return, workers headed home, only to turn back to the office after sitting in traffic even worse than usual.

All outbound flights from San Diego International Airport were stopped. Some inbound flights were allowed, but Rebecca Bloomfield, an airport spokeswoman, said many were diverted to other airports.

Jason Bump, who turned 38 on Thursday, had hoped to head home early to celebrate his birthday. But he was stranded in the parking lot of his office in Carlsbad, Calif., throwing a football around with co-workers.

“I almost took the day off for my birthday,” he said. “I probably should have.”

San Diego Gas and Electric said the blackout seemed to have originated in northern Arizona. It said it was working to repair the two lines that had been “tripped off,” resulting in the failure throughout the region, but made no estimate of when service would be restored.

Late in the afternoon, the San Diego Police Department reported many calls about the blackout, but no other major problems related to the power failure.

San Diego residents called friends around the country for updates on the blackout, as rumors flew about what was going on. Some people lucky enough to make it home organized neighborhood gatherings. Carrie Sandys bought the last of the ice from a corner store and planned a barbecue with her neighbors to use up their produce, now that their refrigerators were out.

They had meat and beer on ice, but still, they could not turn on the opening game of the National Football League season.