LOS ANGELES — A power failure swept through Southern California on Thursday afternoon, leaving more than a million people without electricity in an area that extended from the coast east to Arizona and south into Mexico.

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.