At least 13 people have died and three million were left without power after violent storms pummelled the eastern United States.
At least six of the dead were killed in Virginia, including a 90-year-old woman who was asleep in bed when a tree slammed into her home.
Two young cousins in New Jersey were killed when a tree fell on their tent while camping. Two people were killed in Maryland, while single deaths were reported in Ohio, Kentucky and Washington.
Energy officials have said the power outages would not be repaired for several days, likening the damage to the effects of a serious hurricane.
Emergencies were declared in Maryland, West Virginia, Ohio, the District of Columbia and Virginia, where governor Bob McDonnell said the state had its largest non-hurricane outage in history, as more storms threatened. “This is a very dangerous situation,” Mr McDonnell said.
In West Virginia, 232 Amtrak passengers spent Friday night on a train that was blocked on both sides by trees that fell on the tracks, and they were waiting for buses to pick them up on Saturday.
In Illinois, storm damage forced the transfer of dozens of maximum-security, mentally ill prisoners from one prison to another.
In some suburbs of Washington, emergency call centres were out of service, with residents told to call local police and fire departments.
Huge trees fell across streets in Washington, crushing cars underneath, while others fell onto the fairway at the AT&T National golf tournament in Maryland. Mobile phone and internet service was erratic, petrol stations shut down and residents were urged to conserve water until power returned to sewage plants.
The outages were especially dangerous because they left the region without air conditioning in an oppressive heat. Temperatures soared in Baltimore and Washington, reaching 40C on Friday.
On Saturday night, train passengers stranded near rural Prince, West Virginia, were loaded into buses after they got stuck at 11pm the previous evening, said Amtrak spokesman Steve Kulm.
Mr Kulm said the train bound from New York to Chicago had power, so lights and air conditioning were working. He said that since it was a long-distance train, it was stocked with food and crew members were able to get to town to buy more.
The storm caused damage from Indiana to New Jersey, although the bulk of it was in West Virginia, Washington and suburban Virginia and Maryland.
Illinois prisons officials transferred 78 inmates from a facility in Dixon to the Pontiac Correctional Centre after storms caused significant damage on Friday night, Department of Corrections spokeswoman Stacey Solano said.
No-one was injured. Generators are providing power to the prison, which is locked down, confining remaining inmates to their cells.
Utility officials said it could take at least several days to restore power to all customers because of the sheer magnitude of the outages and the destruction. Winds and toppled trees brought down entire power lines, and debris has to be cleared from power stations and other structures.
The storm that whipped through the region on Friday night was called a derecho, a straight line wind storm that sweeps over a large area at high speed.
It can produce tornado-like damage. The storm, which can pack wind gusts of up to 90mph, began in the Midwest, passed over the Appalachian Mountains and then drew new strength from a high pressure system as it hit the south-eastern US, according to meteorologists.