Sweltering late-summer heat and the loss of a key transmission line Thursday forced power officials in Southern California to impose rolling blackouts, leaving as many as half a million people without power for an hour at a time, officials said.

The California Independent System Operator (search), which operates the state's electric grid, declared a transmission emergency at 3:57 p.m., said Gil Alexander, a spokesman for SoCal Edison (search).

The ISO ordered the utility to reduce its demand, prompting blackouts in parts of Fontana (search), La Puente, Cathedral City, Huntington Beach, Long Beach and Ontario, Alexander said.

The utility planned to scatter the blackouts to lessen the impact, he said. Blackouts were expected to last about an hour before being shifted to other areas.

SoCal Edison has about 13 million customers in more than 400 Southern California cities and communities.