A massive blackout caused by "human failure" left nearly 5 million people without power in parts of California, Arizona and Mexico yesterday.

The outage was apparently triggered by an employee who carried out a procedure at a substation in Arizona.

The outage snarled traffic on Southern California freeways, knocked out water supplies in parts of San Diego County and Tijuana and sent some elderly residents to emergency rooms.

San Diego International Airport cancelled all outbound flights, traffic came to a standstill as the city's street lights stopped working and about 70 people had to be rescued by the city's fire department from stalled elevators.

San Diego schools were ordered closed until Monday as utilities could not guarantee they would be able to turn on the lights in classrooms.

"There was a very major outage, a region-wide outage," San Diego Gas and Electric President Mike Niggli said. "There's no doubt this has never happened before to our system."

But police in California's second-largest city, located between Los Angeles and the Mexican border, reported no major problems, and hospitals successfully switched to back-up power, the Scripps Health chain said.

The ill-fated procedure in Arizona first caused the failure of a high-power line supplying electricity to Southern California before unleashing a domino effect across the southwest, officials said.

That in turn led to a blockage at California's San Onofre nuclear energy plant, a second major source of power to the San Diego area, San Diego Gas and Electric said.

San Diego Gas and Electric said in a tweet that all 1.4 million of its customers in the San Diego area were without power.

Blackouts also affected 3.5 million people in Baja California, according to local emergency services and state authorities.

The city of Yuma, Arizona, reported that more than 50,000 people had lost power.

''There appears to be two failures here - one is human failure and the other is a system failure. Both of those will be addressed," said Damon Gross, a spokesman for Arizona utility APS.

By early evening, crews had restored service in the section of the line that triggered the massive event and had begun to restore power to parts of San Diego County.

Electricity returned late yesterday to the central San Diego neighbourhood of Normal Heights, where many families earlier in the evening had embraced the darkness by throwing outdoor barbecue parties on their front lawns.

By 11pm, San Diego Gas and Electric reported that power was restored to 165,000 customers in San Diego and Orange counties.

However, the utility warned that all power would not be restored overnight and urged customers to conserve energy.