a maintenance worker’s blunder caused a massive power cut that left more than six million people in the U.S. and Mexico in darkness.

The failure brought America’s eighth-largest city, San Diego in California, to a virtual standstill for hours.

Two nuclear reactors were taken offline, flights were grounded, people were trapped in lifts and on fairground rides and massive traffic jams spread as power workers struggled to get the lights back on.

At first there were fears the blackout may have been part of a plot timed to coincide with the tenth anniversary of the 9/11 terrorist attacks. But the explanation turned out to be simpler - a worker in Arizona accidentally triggered the cut while replacing a piece of monitoring equipment at a power substation.

The error, on a high-voltage power line linking Arizona and San Diego, caused a cascading series of electrical grid failures stretching through southern California and into Mexico.

A spokesman for APS, Arizona’s largest electricity provider, said: “This was not a deliberate act. The employee was just switching out a piece of equipment that was problematic.”

There was mayhem on the streets on Thursday evening with scores of rush-hour traffic accidents and San Diego’s trolley rail system was shut down.

People were trapped on rides at the city’s Sea World and Legoland in nearby Carlsbad.

The San Onofre nuclear power plant in California went offline as it was programmed to do if there is a disturbance with the power grid, but a spokesman insisted there was no danger to the public or workers.

Hospitals and police stations switched to back-up generators to carry on working.

Officials said very hot weather - with temperatures hitting about 115f in some areas - may have played a part in the shutdown.

The blackout began at about 4pm on Thursday and power to most areas was back on by yesterday morning.