Seven states along the east coast of the US, from North Carolina to Connecticut, have declared emergencies ahead of Hurricane Irene's arrival.
The first hurricane of the Atlantic season weakened slightly early on Friday to a category two storm, with winds of up to 110mph (175km/h).Mandatory evacuations have been ordered in parts of Delaware, Maryland, New Jersey and North Carolina.
Irene has caused havoc in the Caribbean and could do the same in the US.
At 0800 EDT on Friday (1200 GMT on Friday), the storm was 375 miles south-south-west of Cape Hatteras, North Carolina, said the US National Hurricane Center.
Heightened waves Irene could affect up to 65 million people in major cities along the east coast from Washington to Baltimore, Philadelphia, New York and Boston.
"We're going to have damages, we just don't know how bad," Craig Fugate, the 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."
States of emergency have been declared in North Carolina, Maryland, Virginia, Delaware, New Jersey, New York and Connecticut.
President Barack Obama also declared an emergency in North Carolina, where Irene is due to make landfall first, on Saturday afternoon.
The move allows greater co-ordination between state and US federal disaster management authorities.
Forecasters said Irene could strengthen slightly before its expected arrival in North Carolina on Saturday. It is then expected to weaken as it moves up the east coast, diminishing in strength by Sunday.
Heightened waves began hitting North Carolina's Outer Banks early on Friday.
In Washington DC, which is under a tropical storm watch, Sunday's scheduled dedication of the newly-opened memorial for Martin Luther King Jr - which President Obama had been expected to attend - has been postponed until at least September.
Huge wind span Irene boasts hurricane force winds extending 80 miles from its centre, and tropical storm winds reaching up to 295 miles from the eye.
US authorities are warning of dangerous storm-surge seas, high waves and rip-tide currents along the coast stretching up from North Carolina, through Chesapeake Bay and Maryland's Eastern Shore.
Further north still, New Jersey, New York City and Long Island - densely populated areas not usually prepared for hurricanes - are expected to take a buffeting.
Hundreds of thousands of people in low-lying and beach-front areas of the nation's biggest city, New York - which has not seen a hurricane for decades - have been advised to be prepared to move elsewhere ahead of Irene's anticipated arrival on Sunday.
Much of New York's subway system and other infrastructure is underground and could be flooded, officials have noted.
Amtrak, America's passenger rail service, announced it was cancelling train travel south of Washington on the east coast, and airlines predicted widespread disruptions to air travel at the weekend.
In Virginia, the US Navy ordered its Second Fleet to leave Norfolk Naval Station in Virginia on Thursday morning and head out to sea.
"The forecasted destructive winds and tidal surge is too great to keep the ships in port," said Vice Adm Daniel Holloway, the fleet's commander.
North Carolina emergency officials have extended evacuation orders to include more than 200,000 tourists and residents in three coastal counties.
Visitors to the region have been leaving the area, while residents are preparing to ride out the storm by stocking up on food, water and fuel.
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