SAN DIEGO - A massive power outage hit southern California and parts of Arizona and Mexico, affecting well over a million people, San Diego Gas and Electric said on Thursday.

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.

San Diego stoplights shut off and the utility warned customers that power might not be restored overnight.

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