Hurricane Irene closed in on the U.S. Atlantic coast on Friday, triggering emergency preparations that included unprecedented mandatory evacuations in New York City as the massive storm approached.

As Irene careened north, rain and tropical storm force winds began sweeping with mounting force across the North Carolina coast.

Washington and states from the Carolinas through Maine declared emergencies due to Irene, a nearly 600 mile-wide hurricane that forced 55 million Americans on the eastern seaboard to prepare for and that experts say could cause billions of dollars in damages.

President Barack Obama said the impact of the storm, which is unusually large, could be extremely dangerous and costly for a nation that still remembers destructive Hurricane Katrina in 2005.

All indications point to this being a historic hurricane, Obama said.

Hundreds of thousands of residents and vacationers were evacuating from Irene's path, starting in east North Carolina where the hurricane was expected to make landfall on Saturday morning.

A quarter of a million New Yorkers were ordered to leave homes in low-lying areas of Manhattan, Brooklyn and Queens, as authorities prepared for dangerous storm surge and flooding on Sunday in the city and farther east on Long Island.

Some New York hospitals in flood-prone areas were already evacuating patients, and New York's mass transit system, which carries 8.5 million people a weekday, was due to start shutting down around noon (1600 GMT) on Saturday.

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, Mayor Michael Bloomberg told a news conference.

As U.S. authorities ramped up preparations to cope with a potential major natural disaster on the densely populated East Coast, U.S. airlines canceled more than 3,000 flights and moved airplanes out of Irene's path.

Officials were taking every precaution with Irene because they remember all too well how Hurricane Katrina in 2005 swamped New Orleans, killing up to 1,800 people and causing $80 billion in damage.

Defense Secretary Leon Panetta said the military stood ready to aid in the response to Irene, with more than 100,000 National Guard forces available if needed in East Coast states.

Coastal communities stocked up on food and water and tried to secure homes, vehicles and boats. Cities, ports, hospitals, oil refineries and nuclear plants activated emergency plans.

We've been through about four or five (hurricanes), but this looks like it'll be the worst, Henry Burke, a vacation homeowner in Atlantic Beach, North Carolina, told Reuters.

It's always fun to see what a hurricane stirs up, said Mike Fox, 51, who said he planned to ride out the storm with his wife Sheri in their home in vulnerable Morehead City, North Carolina, near the site of Irene's expected U.S. landfall.

Where we're living we really don't know how that place is going to hold up, Fox said.

U.S. federal and state leaders, from Obama downward, urged the millions of Americans in the hurricane's path to prepare and to heed evacuation orders if they received them.  

WIND FIELD HUGE

Irene weakened early on Friday to a Category 2 hurricane from a 3 on the five-step Saffir-Simpson scale, but it still was carrying winds of up to 100 miles per hour.

It was expected to remain a hurricane as it sweeps up the mid-Atlantic coast over the weekend. The Miami-based hurricane center said it could dip below hurricane strength before reaching New England, but its impact would not vary much.

At 8 p.m. (midnight GMT), Irene's center was churning north-northeast ward 235 miles south-southwest of Cape Hatteras, North Carolina.

Irene, the first hurricane of the 2011 Atlantic season, had already caused as much as $1.1 billion in insured losses in the Caribbean this week, catastrophe modeling company AIR Worldwide said, with more losses expected to come.

The National Hurricane Center said hurricane force winds extended outward up to 90 miles from Irene's center, while tropical storm force winds extended out up to 290 miles, giving the storm a vast wind field width of nearly 600 miles.

The wind field is huge, NHC Director Bill Read told Reuters Insider.

In earlier comments, Read said Irene, which will be the first significant hurricane to affect the populous U.S. Northeast in decades, would lash the eastern seaboard with tropical storm-force winds and a huge swath of rain from the Carolinas to New England.

WATCHING THAT BIG WHITE SWIRL  

Wall Street firms scrambled to raise cash into early next week in case Irene caused major disruption in trading. Bond trading volume dropped precipitously by noon on Friday.

Traders were watching that big white swirl on their television sets, said Guy LeBas, chief fixed income strategist at Janney Montgomery Scott in Philadelphia.

Northeast oil, natural gas and power facilities also made preparations.

Brent crude oil futures rose in choppy trade on Friday as Irene targeted the East Coast and traders weighed comments by Federal Reserve Chairman Ben Bernanke on the economy.

Benedict Willis III, director of floor operations for investment banking boutique Sunrise Securities, said the New York Stock Exchange had a responsibility to open on Monday after the storm passes because millions of investors will be relying on it for stock prices.

But if the waters rise this high, he said, gesturing at the buzzing trading floor, then it's a bigger problem than I can handle. My name's not Noah.

Irene will be the first hurricane to hit the U.S. mainland since Ike pounded Texas in 2008.

In Washington, Irene forced the postponement of Sunday's dedication ceremony for the new memorial honoring civil rights leader Martin Luther King Jr. Tens of thousands of people, including the president, had been expected to attend.

Flooding from Irene killed at least one person in Puerto Rico and two in Dominican Republic. The storm knocked out power in the Bahamian capital, Nassau, and blocked roads with trees.