Southern California and Mexico massive blackout . More than 4 million without power.

San Diego blackout: power restored for all 1.4 million SDG&E customers.

More than 4 million people across a large swath of Southern California and Mexico have been left without power on Thursday after an utility worker doing maintenance near Yuma, Arizona, triggered a massive blackout that jammed roads, closed schools and businesses, grounded planes.

After the utility worker triggers a chain reaction that reaches from Mexico to Orange County, many offices have closed and employees endured gridlock getting home because traffic lights were out.

According to authorities, it was noticed an increase in fender-benders in some areas as drivers tried to navigate the roads.

Many people were trapped in elevators and on rides at Sea World in San Diego and Legoland in Carlsbad.

At hospitals, all emergency rooms have been switched to backup generators, while outgoing flights from San Diego have been canceled for several hours.

Customers jammed those stores that remained open, stocking up on ice and candles as utility company officials warned that power may not be restored until late Friday.

Authorities canceled classes for Friday at most colleges and schools in San Diego and surrounding communities.

Michael Niggli, president and COO of San Diego Gas & Electric said:

“Get ready to be in the dark. Get your emergency precautions ready.”

The blackout was triggered by a mishap on a high-voltage power line linking Arizona and San Diego, causing a cascading series of electrical grid failures stretching into Southern California.

APS, Arizona’s largest electric utility, said a worker was doing maintenance on lines at a nearby substation when the blackout occurred.

An APS spokesman said in a statement:

“The outage appears to be related to a procedure an APS employee was carrying out in the North Gila substation.

“Operating and protection protocols typically would have isolated the resulting outage to the Yuma area. The reason that did not occur in this case will be the focal point of the investigation into the event, which already is underway.”

“Despite temperatures that reached 100 F in San Diego and Imperial counties, excessive electricity demand didn’t appear to be a factor in the power loss,” said Stephanie McCorkle, a spokeswoman for the California Independent System Operator, the agency that oversees most of California’s electrical grid.

“It was not a case of a high-demand day,” McCorkle said.

“The operating reserves were fine.”

According to officials, utility crews were scrambling to restore some power by tapping into local energy sources at gas-fired plants in Escondido and Otay Mesa.

Power had been restored to some communities in Orange and Imperial counties by Thursday night, but power wouldn’t be fully restored until Friday, officials said.

Regions of Baja California as well as Arizona were also without power.

The power outage caused disarray across the region, interrupting Amtrak trains and trolley service in San Diego and causing gasoline station closures.

Several sewage pumps failed during the blackout, sending effluent into San Diego Bay.

Residents of a nursing home in Indio in Riverside County were evacuated when their facility lost power, and county officials opened a cooling center. At San Diego International Airport, dozens of travelers were left stranded when their flights were canceled.

In downtown San Diego’s usually bustling Gaslamp district, most businesses were closed. In Oceanside, people shopped in a dark 99-cent store, and some cashiers tallied bills by calculator.

San Diego County hospitals were operating on backup power, but conditions were challenging staff at Scripps Hospital in La Jolla, which had 220 patients when the power went out.

“We are in full disaster mode,” said Gary Fybel, chief executive of the hospital.

“We completed all surgeries that were underway. We did not take on new surgeries, [and] delayed nonessential treatments.”

The hospital staff conducted its regular meetings by flashlight just outside the main entrance of the hospital. Many hospital rooms were illuminated with power produced by generators.