(Reuters) - 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.