A massive power outage triggered by human error left well over a million people without electricity in Southern California and parts of Arizona and Mexico on Thursday, power utilities said, warning service might not be restored until the next day.

An ill-fated procedure by an Arizona power utility employee in a small town unleashed a chain of events which brought down power in a large swath of the Southwest, the Arizona utility said. A high-power line supplying electricity to Southern California failed first.

Minutes later, that led to a blockage at California's San Onofre nuclear energy plant, a second major source of power to the north of San Diego, which shut down, San Diego Gas and Electric said.

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

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

Local utility San Diego Gas and Electric said in a tweet that all 1.4 million of its customers were without power, while nearby utility Southern California Edison said the blackout had forced its two-unit San Onofre nuclear plant offline.

A spokeswoman said the plant had shut down safely.

Arizona utilty APS said an employee near the small city of Yuma had carried out a procedure at a local substation which triggered the outage. System safeguards then failed to stop the problem spreading.

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

URGED TO KEEP OFF ROADS

San Diego Gas and Electric has restored transmission to two major transmission lines involved in the crisis, but the company warned customers that power might not be restored overnight. Prepare to stay home tonight without power, the utility, owned by Sempra, told customers.

Stuck without refrigeration, employees at the Cardiff Seaside Market, a grocery and specialty food store in the town of Cardiff-by-the-Sea, north of San Diego, started grilling their inventory of fresh steaks and tuna in the parking lot and selling it cooked to passersby for cash.

Meanwhile, a line of about 50 customers waited at the front door for their turn to be led inside by a clerk to do their shopping in groups of two or three at a time.

It's real hectic, there's lines everywhere now. But the customers are happy, everyone's patient, everyone's in a good mood, and we're serving them as quickly as we can, manager John Shamam, 33, said as he served up a plate of tuna.

Many of the Tweets from San Diego residents revolved around air conditioning. I'm going to die of heat in this house with no AC! wrote Ashleigh Marie. What am I supposed to dooo.

But San Diego resident Kiersten White tweeted that the power outage makes me glad I don't have air conditioning to begin with ... nothing to miss!

Other people were having a harder time. Trapped outside of our rooms at the hotel, tweeted Rob Myers, visiting from Washington. Other members of his party got trapped in an elevator, he wrote.

Blackouts hit Mexico's Northern Baja California state in the afternoon, knocking out power to hundreds of maquiladora export assembly plants in the sprawling industrial powerhouse of Tijuana, south of San Diego.

The blackouts knocked out stop lights at intersections across Tijuana, causing traffic snarl ups, and also cut power to hospitals and government offices. The border crossing at Otay Mesa was closed to all but pedestrian traffic.