MIchele Le September 9, 2011 12:18 AM EDT

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.