SAN DIEGO — SAN DIEGO — Millions of residents in San Diego County are expected to spend the night in the dark after a major transmission line supplying power to Southern California went down at about 3:38 P.M., leaving the sprawling region without power.

The sudden outage plunged the region into chaos as people were stuck in elevators, traffic signals were out at almost all of the intersections in the county, causing major traffic jams. Some hospital patients had to be moved from hospitals without power.

A worker conducting a procedure at a switching plant in Central Arizona where major electricity transmission lines merge triggered the outage, Arizona power officials said.

Some electricity will be restored overnight Thursday into Friday, said Mike Niggli, CEO of San Diego Gas & Electric. Most customers are expected to have power by the end of the day Friday. But some customers may be out into Saturday. Urban areas will be restored before outlying areas.

"That is going to take quite a few hours to do that," Niggli told KOGO AM radio in an interview. "I'm looking at the first customers coming on late tonight."

The outage was exacerbated when workers tried to reconnect the transmission line and it failed.

After that event, the two units at the San Onofre Nuclear Generating Station went offline, which severed the electricity connections from the north. That left the entire SDG&E network without any power, leaving all of San Diego County without electricity. Other areas affected include parts of southern Orange County & Imperial County and northern Baja California, including Tijuana.

Power was reportedly restored in Imperial County by Thursday afternoon.

The major transmission line that supplies electricity from Arizona is back in service, but the connection to San Diego County has not been restored. Workers are now restoring the local San Diego system to the high voltage system, which has to be done in pieces, Niggli said in an interview on KOGO AM radio.

Officials are asking residents to stay at home, leaving the streets clear for emergency personnel. They are also asking people not to use landline telephones, only in case of an emergency. Residents are also urged to restrict water use as pumping stations have lost power and the system is stressed.

We will update as soon as more information is available. For more immediate updates, follow the KPBS News Twitter account: @kpbsnews