An electrical operator's error caused a massive power outage Thursday that swept across Arizona, Southern California and Mexico, left about six million people in the dark and brought several West Coast cities to a standstill, according to a local power company.

The San Diego Gas and Electric (SDGE) said the blackout, which began around 3:40 p.m, could be traced to an employee who removed a piece of monitoring equipment at a power substation in southwest Arizona.

The SDGE said both its power supply lines from Arizona and from the north appeared to fail, and the outage might not be fully restored until Friday.

With the wide-spread electrical power outage impacting most of San Diego and surrounding areas, the California Independent System Operator Corporation (ISO) is declaring a Flex Alert for September 9, 2011, Erin Johnson, a technical assistant from ISO told Xinhua in an Email.

The Flex Alert is in effect statewide as conservation in both Southern and Northern California will ease the strain on the power grid.

The power line break "caused our line from AZ and from the north of our region to both trip off," the SDGE posted on its Twitter account.

"No cause is determined at this time," another Twitter post from the utility said.

The Southern California blackout was triggered after a 500-kilovolt high-voltage line from Arizona to California tripped out of service, David Song, a spokesman from Southern California Edison told Xinhua.

The blackout led to a shutdown of two reactors at the San Onofre nuclear power plant in Southern California and there was no danger to the public or to workers there, Song said.

The widespread outages have darkened homes and stores and caused massive traffic jams across Southern California including some important public facilities, Michael Workman, a San Diego County spokesman said.

"All outgoing flights from San Diego International Airport have been suspended, and the airport is currently running on generators", Rebecca Bloomfield, a spokesman of the airport told Xinhua.

San Diego Gas & Electric Co. also sent out a barrage of alerts on its Twitter feed, warning customers " for safety never plug a portable generator into any electric outlets" and "Remember to turn off air conditioners to prevent them from unexpectedly coming on when the power is restored."

Two million more people were reported to be without power in Mexico, local Tv reported.

The outage came after a more severe blackout in 2003 darkened a large swath of the Northeast and Midwest. More than 50 million people were affected in that outage.