Hurricane Irene opened its assault on the Eastern Seaboard today by lashing the North Carolina coast with wind as strong as 115 mph and pounding shoreline homes with waves.
Further north in New York City and Philadelphia, authorities readied a massive shutdown of trains and airports, with two million people ordered out of the way.
The centre of the storm, which was estimated to be some 500 miles wide, passed over North Carolina's Outer Banks for its official landfall just after 7.30am EDT (11.30am Irish time).
The hurricane's vast reach traced the East Coast from Myrtle Beach, South Carolina, to just below Cape Cod. Tropical storm conditions battered Virginia, Maryland and Delaware, with the worst to come.
By afternoon, forecasters said Irene's effects could be felt as far north as Canada even after it weakens. A tropical storm warning extended from the US border to Nova Scotia's southern coast.
Irene weakened slightly, with sustained winds down to 85 mph from about 100 mph a day earlier, making it a category one, the least threatening on the scale.
The National Hurricane Centre reported gusts of 115 mph and storm-surge waves as high as seven feet.
The first death from the storm was reported in Nash County, North Carolina, outside Raleigh, where emergency officials said a man was crushed by a large limb that blew off a tree.
Hurricane-force winds arrived near Jacksonville, North Carolina, at first light, and wind-whipped rain lashed the resort town of Nags Head. Tall waves covered the beach, and the surf pushed as high as the backs of some of the houses and hotels fronting the strand.
"There's nothing you can do now but wait. You can hear the wind and it's scary," said Leon Reasor, who rode out the storm in the Outer Banks town of Buxton. "Things are banging against the house. I hope it doesn't get worse, but I know it will. I just hate hurricanes."
At least two piers on the Outer Banks were wiped out, the roof of a car dealership was ripped away, and a hospital in Morehead City that was running on generators. In all, nearly half a million people were without power on the East Coast.
The Red Cross had opened five shelters in North Carolina by Saturday afternoon, with more than 700 people registering to stay. At the shelter on the Elizabeth City State University campus, children shrieked and played while their parents watched the storm unfold.
Latoya Bryant watched her children play, saying she told them they could go home once the storm was over.
"They want to play in the rain, though," Bryant said. "You can't play in this rain."
In the Northeast, unaccustomed to tropical weather of any strength, authorities made plans to bring the basic structures of travel grinding to a halt. The New York City subway, the largest in the United States, was making its last runs at noon, and all five area airports were accepting only a few final hours' worth of flights.
The New York transit system carries five million people on weekdays, fewer on weekends, and has never been shut for weather. Transit systems in New Jersey, Baltimore and Philadelphia also announced plans to shut down. Washington declared a state of emergency. Days earlier, buildings had been evacuated after an earthquake rattled the D.C. area.
New York City ordered 300,000 people to leave low-lying areas, including the Battery Park City neighbourhood at the southern tip of Manhattan, the beachfront Rockaways in Queens and Coney Island in Brooklyn. But it was not clear how many people would get out, or how they would do it.
"How can I get out of Coney Island?" said Abe Feinstein, 82, who has lived for half a century on the eighth floor of a building overlooking the boardwalk. "What am I going to do? Run with this walker?"
Authorities in New York said they would not arrest people who chose to stay, but Mayor Michael Bloomberg warned on Friday: "If you don't follow this, people may die."
Streets and subway cars were much emptier than on a typical Saturday morning. On Wall Street, sandbags were placed around subway grates nearest the East River, which is expected to surge as the worst hits New York.
The city's largest power company said it could cut power to some neighbourhoods if the storm causes serious flooding. Salt water can damage power lines, and cutting power would speed repairs.
In all, evacuation orders covered about 2.3 million people, including one million in New Jersey, 315,000 in Maryland, 300,000 in North Carolina, 200,000 in Virginia and 100,000 in Delaware. Authorities and experts said it was probably the most people ever threatened by a single storm in the United States.
Airlines said 8,300 flights were cancelled, including 3,000 today. Greyhound suspended bus service between Richmond, Virginia, and Boston. Amtrak cancelled trains in the north east for Sunday.
Forecasters said the core of Irene would roll up the mid-Atlantic coast Saturday night and over southern New England on Sunday.
In Newport News, Virginia, a city spokeswoman said an 11-year-old boy was killed when Irene's winds sent a tree crashing through his apartment building. The boy's mother was able to make it out of the apartment uninjured. Winds were gusting well above 60 mph in the area.
A Virginia Department of Emergency Management spokesman said a tree fell on a car in Brunswick County and killed someone inside the vehicle.