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More than two million people on the US east coast have been told to evacuate their homes as Hurricane Irene nears, packing winds of 90mph (150km/h).
The mayor of New York has ordered an unprecedented evacuation of a quarter of a million people living in low-lying parts of the city.Seven states from North Carolina to Connecticut have declared emergencies ahead of Irene's arrival.
US President Barack Obama has warned Irene could be "a historic hurricane".
He has urged people in the projected path of Hurricane Irene - the first hurricane of the Atlantic season - to take precautions.
"Don't wait, don't delay. We all hope for the best, but we have to be prepared for the worst. All of us have to take this storm seriously," he said on Friday, before cutting short his holiday in Martha's Vineyard, an island on the Massachusetts coast, a day early to head back to Washington.
The National Hurricane Center has downgraded Irene from a category two to a category one hurricane but says it is still packing hurricane-force winds of 90mph (150km/h) that extend outwards some 90 miles (150km). Tropical-force winds extend as far as 290m (465km).
The NHC says Irene is expected to weaken in strength after it hits the coast of North Carolina later on Saturday morning, but is forecast to remain a hurricane as it moves north along the mid-Atlantic coast on Sunday.
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President Obama: "If you're in the way of this hurricane you should be preparing now"
Forecasters have warned of "extremely dangerous" storm surges in parts of North Carolina that could raise water levels by as much as 11 ft (3.35m).
"Stores are busy, petrol stations are running dry but thankfully I prepared myself last night with supplies," said Alex Schlesinger of Virginia Beach in the neighbouring state of Virginia, also in the hurricane's path.
Grounded flights Tens of thousands of people were also on the move in parts of Delaware, Maryland, New Jersey and New York City after mandatory evacuations were ordered for people living in low-lying areas.
Parts of New York City affected include the financial district around Wall Street in Manhattan. Hospitals in affected areas of the city had begun evacuating patients.
"We've never done a mandatory evacuation before and we wouldn't be doing it now if we didn't think this storm had the potential to be very serious," said Mayor Michael Bloomberg.New York Governor Andrew Cuomo said the state's transport network, including the New York City subway, would close from midday (16:00 GMT) on Saturday.
Airports operating by the Port Authority of New York and New Jersey - including JFK, La Guardia and Newark - will close to arriving flights at the same time.
However, the airports will remain open for departing flights until further notice.
In Washington DC, Sunday's dedication of the new memorial for Martin Luther King Jr - which President Obama had been expected to attend - has been postponed until at least September.
Manhattan, like other parts of the east coast, braces itself for Hurricane Irene Amtrak, the US rail network, announced it was cancelling services between Washington and Boston from Saturday, having already suspended operations south to Virginia and beyond.
The Pentagon has loaded 200 trucks with emergency supplies, and 100,000 National Guard troops are on standby, the BBC's Steve Kingstone in New York reports.
The American Red Cross said it was preparing dozens of emergency shelters along the east coast.
The eastern seaboard is the most densely populated corridor in the US, with more than 65 million people living in major cities along the coast from Washington to Baltimore, Philadelphia, New York and Boston.
States of emergency have been declared in North Carolina, Maryland, Virginia, Delaware, New Jersey, New York and Connecticut.
"We're going to have damages, we just don't know how bad," Craig Fugate, head of the US Federal Emergency Management Agency, told the Associated Press news agency.
"This is one of the largest populations that will be impacted by one storm at one time."
If Irene hits New York and New England at category two, it will be the region's strongest storm since Hurricane Bob glanced off Massachusetts in 1991, and Hurricane Gloria, which caused extensive damage to New York City in 1985.
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