Hollywood and other parts of Los Angeles were left without power for two hours on Monday after workers accidentally overloaded a power line.
Drivers found themselves in huge jams around the city
Police declared an emergency as traffic lights went blank and lifts stopped. The city's international airport had to switch to emergency power.
After initial confusion and fears of a terror attack, the fault was traced to a power surge from a receiving station.
Al-Qaeda has reportedly made a recent threat to attack the Californian city.
Ron Deaton, general manager of the Los Angeles Department of Water and Power, said city maintenance workers had accidentally cut a line at a receiving station.
They then connected it to another line "that was not expecting that amount of electricity".
The surge which hit at 1237 local time (1937 GMT) disrupted service to more than half of the Los Angeles region.
Police and fire services were put on "tactical alert", meaning staff could not leave work when their shifts were over, and only life-or-death emergency calls would be answered.
The biggest impact seemed to be on the roads, where chaos ensued as drivers tried to negotiate the city without functioning traffic lights, but there were no immediate reports of injuries.
The US has suffered severe strains on electricity supply in recent years.
In 2003, 50 million homes in New York and the north-eastern US lost power.
California had to restrict its supply during a 2001 power crisis in the state, imposing rolling cuts.