LOS ANGELES (AP) – Sweltering heat and the loss of power from a key transmission line Thursday forced the utility serving Southern California to impose rolling blackouts, leaving as many as half a million people without power for about half an hour, officials said.

The California Independent System Operator, which operates the state’s electric grid, declared a transmission emergency at 3:57 p.m., said ISO spokeswoman Stephanie McCorkle.

About 30 minutes later, power was being restored to people subjected to the blackouts, she said.

It marked one of the most serious disruptions since the California power crisis in 2000 and 2001, when high demand, high wholesale energy costs, transmission glitches and a tight supply caused widespread problems including blackouts.

The ISO ordered Southern California Edison to reduce demand Thursday, prompting the temporary blackouts in areas of Fontana, La Puente, Cathedral City, Huntington Beach, Long Beach and Ontario, said Gil Alexander, a spokesman for the utility.

The utility scattered the outages to those points east and south of Los Angeles.

If the problem had persisted, officials said they would have shifted the blackouts to other areas on a rotating basis.

Temperatures that hovered around 100 in inland areas and reached 94 in Los Angeles had created increased demand of about 1,500 megawatts. A megawatt is enough power to serve about 750 homes.

The demand crisis was exacerbated by the sudden loss of power from a transmission line originating in southern Oregon, officials said.

A transformer in California that converts the power took itself offline automatically at 3:56 p.m. when an oil flow alarm went off, said Carol Tucker, a spokeswoman for the Los Angeles Department of Water and Power, which co-owns the line.

The converter station, in Sylmar about 25 miles north of downtown Los Angeles, is capable of distributing 2,250 megawatts of power. It was operating at half capacity Thursday evening, and an inspection was under way to determine if the transformer could be fixed, Tucker said.

The emergency order from the ISO caused SoCal Edison to reduce demand by 800 megawatts throughout its territory. The ISO asked San Diego to shed 100 megawatts.

Northern California was not affected by the shutdowns.

SoCal Edison serves about 13 million people in more than 400 Southern California cities and communities.

During the 2000-2001 energy crunch, SoCal Edison and Pacific Gas and Electric lost billions of dollars because of high wholesale prices that the state’s electricity deregulation law barred them from passing on to consumers.

During the crisis, the state’s energy market was also manipulated by traders from companies that included now-bankrupt Enron Corp.

Few problems were reported in communities struck by Thursday’s blackouts.

Long Beach police Officer Greg Schirmer said the utility notified the department in advance of the outage.

“We’ve had rolling blackouts in the past and we’ve been fortunate enough not to have any issues,” he said.

Rozanne Adanto, community services director in La Puente in the east San Gabriel Valley, said the city received an e-mail notifying it of possible power outages at 4:10 p.m.

“I did notify staff to keep saving their documents on the computer,” she said.