Volunteer police directed traffic in Cardiff as traffic lights quit due to the power cut [Reuters]

A power outage triggered by human error has left at least two million people without electricity in the US states of California and Arizona, as well as in Mexico's northwestern state of Baja California.

Power began to be restored by late evening on Thursday but energy companies warned that 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 across a large swath of southwestern states, the Arizona energy company APS said.

"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.

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 cancelled all outbound flights, and traffic came to a standstill as the city's street lights went out. About 70 people had to be rescued by the city's fire department from stalled elevators.

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.

Stuck without refrigeration, employees at a 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 passers by 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.

Residents' main concerns appeared to be the lack of air conditioning, with California currently gripped by a heatwave with temperatures over 40 degrees Celsius.

"It feels like you're in an oven and you can't escape,'' Rosa Maria Gonzales, a spokeswoman with the Imperial Irrigation District in California's sizzling eastern desert, said.

The US-Mexico border was cloaked in darkness and police on both sides sent reinforcements into border cities.