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  (Photo: Reuters/Jonathan Ernst)
  Storm-damaged trees litter the east lawn of the U.S. Capitol in Washington June 30, 2012. Wind gusts clocked at speeds of up to 79 mph were reported in and around the U.S. capital, knocking out power to hundreds of thousands of homes in the Washington, D.C., area.

A heat wave in the eastern U.S. was expected to continue on Sunday after violent storms killed at least 12 people and left over 3 million without power. Emergencies were declared in Maryland, West Virginia, Ohio, the District of Columbia and Virginia.

Overnight storms unleashed hurricane-force winds across a 500-mile stretch of the mid-Atlantic region, Reuters reported.

Storm-related incidents killed six people in Virginia, two in New Jersey, two in Maryland, and one in Ohio. And heat was blamed for the deaths of two brothers aged 3 and 5 in eastern Tennessee.

It may take up to a week for power officials to repair the outages in some areas, even as utilities in Ohio, Virginia and Maryland described damage to power grids as catastrophic.

Bob Spieldenner, a spokesman for the Virginia Department of Emergency Management, said in a statement that more than 1 million customers were left powerless in the worst outage not linked to a hurricane in the state's history.

"This is on par with Hurricane Irene," Maryland Gov. Martin O'Malley said.

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"This was a storm that obviously came upon us very quickly, without a great deal of notice, and the devastation that was caused is very significant," New Jersey Gov. Chris Christie said.

About 1 million homes lost power in Ohio, 800,000 in Maryland, 614,000 in West Virginia, 206,000 in New Jersey, and 135,000 in Indiana.

Heat advisories remained in effect on Sunday across the southeast and lower half of the Mississippi valley, according to the National Weather Service. "It is very unsafe outdoors for those susceptible to these extreme conditions," it said in a statement.

President Barack Obama authorized the Federal Emergency Management Agency (FEMA) to coordinate all disaster relief efforts in Ohio.

In West Virginia, felled trees on both sides of a train stranded 232 Amtrak passengers on Friday night, The Associated Press reported. In Illinois, dozens of maximum-security, mentally ill prisoners were transferred from one prison to another.

Emergency call centers in some Virginia suburbs of Washington were out of service, and residents were told to call local police and fire departments.

In Washington, trees felled across streets crunching up cars, and also onto the fairway at the AT&T National golf tournament in Maryland. Cell phone and Internet service and gas stations were also affected.