Lightning struck power lines in Arizona and set off a chain reaction yesterday that left more than a million people in California and Nevada without electricity for up to three hours, power company officials said.

The lightning strikes isolated the two states from a power grid that distributes electricity through several western states. Lights went dark and refrigerators and air conditioners were silent from Northern California to the Mexican border and east to Las Vegas.

The shutdowns started at about 1:30 p.m. and lasted from nine minutes in Southern California to more than three hours in the Las Vegas area, where residents sweated out the failure in 107-degree heat.

Nevada Power Co. officials said about 80,000 customers in the western section of Las Vegas were affected, but casinos escaped the blackout because they are in another part of the city.

In Los Angeles, Orange and Riverside counties, nearly one-third of the 3 million people served by Southern California Edison Co. were hit by a nine-minute blackout beginning at 1:32 p.m. Some 120,000 people in San Diego County also felt the outage.

In Northern California, officials at Pacific Gas & Electric reported scattered outages from Chico, 160 miles north of San Francisco, to San Luis Obispo, 190 miles to the south.