A massive power outage left well over a million people without electricity in Southern California and parts of Arizona and Mexico on Thursday, San Diego Gas and Electric said, warning service might not be restored until the next day.

The utility said in tweets that all 1.4 million of its customers were without power, while nearby utility Southern California Edison said the outage had forced its two-unit nuclear plant offline.

Arizona utilities reported about 56,000 without power since about 3:40 p.m. local time.

Stoplights have shut down, gas stations can't pump gas and mobile phone networks were spotty, University of San Diego Professor Alan Gin said by phone.

Virtually every street light is out, he said.

The San Diego utility warned customers that power might not be restored overnight.

Prepare to stay home tonight without power, the utility owned by Sempra told customers.