WASHINGTON (Reuters) - Blistering heat blanketed much of the eastern United States for the third straight day on Sunday, after violent storms that took at least 15 lives and knocked out power to more than 3 million customers.

Emergencies were declared in Maryland, Ohio, Virginia, West Virginia and Washington, D.C., on Saturday because of damage from storms that unleashed hurricane-force winds across and a 500-mile (800-km) stretch of the mid-Atlantic region.

The storms' rampage came as sweltering temperatures topped 100 Fahrenheit (38 Celsius) in several southern cities, including Atlanta, where the mercury hit an all-time record of 106 degrees (41 Celsius) on Saturday and reached 105 on Sunday.

Over two dozen cities across 10 states set or tied all-time record high temperatures on Friday and Saturday, including Columbia, South Carolina; Knoxville, Tennessee; and Raleigh, North Carolina.

The heat wave continued on Sunday for millions of people from the Plains to the mid-Atlantic. One of the hardest hit cities was Charlotte, North Carolina, where the mercury reached 104 degrees (40 C) on Sunday.

From St. Louis, Missouri, to Washington, D.C., temperatures were forecast to hit more all-time records.

"CATASTROPHIC" DAMAGE TO POWER GRIDS

Thunderstorms and high winds battered eastern North Carolina on Sunday afternoon, leading to three more deaths on top of at least 12 caused by deadly storms in several states on Saturday.

In Pitt County near Greenville, a man was killed when his shed fell on him as he tried to put his golf cart inside, said David Glenn of the National Weather Service.

A couple were killed in neighboring Beaufort County when a tree fell on their golf cart, he said.

More than 40 people were reported injured in Beaufort County and numerous homes were damaged in Pitt County, said Christy Wallace, spokeswoman for the Pitt County sheriff.

After power outages that affected some 15,000 customers, power was restored to most by late Sunday, Greenville Utilities reported.

Powerful storms that brought wind gusts of up to 90 mph on Sunday knocked out power to more than 200,000 Commonwealth Edison customers in Northeastern Illinois and about 138,000 remained without power on Sunday night, the utility said.

Power crews worked on Sunday to restore service to homes and businesses, and officials in some areas said the job could take up to a week. Utilities in Ohio, Virginia and Maryland described damage to their power grids as catastrophic.

Previous storms had left six people dead in Virginia and more than 1 million customers without power, and killed two people in Maryland, officials said.

A falling tree killed two cousins, aged 2 and 7, in New Jersey and heat was blamed for the deaths of two brothers, ages 3 and 5, in Tennessee who had been playing outside in 105-degree (41-degree C) heat

About 489,000 customers remained without power in Maryland on Sunday night, down from more than 1 million without lights and crucial air conditioning earlier on Sunday.

HEATED LABOR DISPUTE

In Ohio, severe storms knocked out power to about 1 million homes and businesses on Friday across two-thirds of the state. Governor John Kasich sought and was granted federal emergency assistance.

Storms had also left about 614,000 customers without power in West Virginia, about 135,000 in Indiana, and at least 206,000 in New Jersey, officials said.

In New York, a heated labor dispute threatened to compound problems posed by the summer heat wave, which has already put an added strain on the electrical grid for New York City and suburban Westchester county.

Power utility Consolidated Edison Inc locked out its unionized workers early on Sunday after contract talks broke down, both sides said, raising the possibility of power cuts.

The company said it had asked to extend negotiations for two more weeks but the union, which had threatened a strike by its 8,500 workers over a new contract, refused. In response, the firm told union members not to report for work on Sunday.

That left managers and any crews the company can hire to fix whatever problems arise as 8.2 million New Yorkers crank up their air conditioners to beat the heat.