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Ann Heisenfelt, Associated Press
Harry Johnson, President and CEO of the Martin Luther King Jr. Memorial Foundation, speaks during a news conference in Washington, Thursday, Aug. 25, 2011, to announce the dedication of Martin Luther King Jr. Memorial has been postponed indefinitely as Hurricane Irene bears down on the East Coast .

MOREHEAD CITY, North Carolina — Tropical storm-force winds from Hurricane Irene began lashing the U.S. East Coast with rain with the potential to cause billions of dollars in damage along a densely populated arc that included Washington, Baltimore, Philadelphia, New York and Boston. At least 65 million people could be affected.

A hurricane warning was in effect Friday from North Carolina all the way to Massachusetts — including for New York City, where more than a quarter-million people were ordered to evacuate ahead of Irene's approach. It was the first hurricane warning issued for New York City in more than two decades.

Officials declared emergencies, called up hundreds of National Guard troops, shut down public transit systems and begged hundreds of thousands of people to obey evacuation orders. Airlines canceled more than 2,000 weekend flights.

The National Hurricane Center said the storm is unlikely to get any stronger and may weaken slightly before reaching land. It said Irene could weaken to a tropical storm before reaching the northern region of New England, but that even below hurricane strength it would be a powerful and potentially destructive storm.

As the storm's outermost bands of wind and rain began to lash islands off the coast of the southern state of North Carolina, authorities in points farther north begged people to get out of harm's way. The hurricane lost some strength but still packed 100 mph (160 kph) winds, and officials in the Northeast, not used to tropical weather, feared it could wreak devastation.

Speaking from Martha's Vineyard Island where he is vacationing, President Barack Obama said all indications point to the storm being a historic hurricane.

"Don't wait. Don't delay," said Obama, who decided to cut short his summer vacation by a day and return to Washington. "I cannot stress this highly enough: If you are in the projected path of this hurricane, you have to take precautions now."

Senior hurricane specialist Richard Pasch of the National Hurricane Center said there were signs that the hurricane may have weakened slightly, but strong winds continued to extend 100 miles (160 kilometers) from its center.

The latest forecasts showed Irene crashing into the North Carolina coastline Saturday, then churning up the Eastern Seaboard and drenching areas from Virginia to New York City before a weakened storm reaches New England.

Rain and tropical storm-force winds of at least 39 mph (63 kph) already were pelting North and South Carolina as Irene trudged north, snapping power lines and flooding streets. Officials warned of dangerous rip currents as Irene roiled the surf. Thousands already were without power. In Charleston, South Carolina, several people had to be rescued after a tree fell on their car, trapping them.

The hurricane forced the president to wrap up his vacation a day early to return to Washington Friday night instead of Saturday afternoon.

Irene's wrath in the Caribbean, including Puerto Rico, the Dominican Republic and the Bahamas, gave a preview of what might be coming to the U.S.: Power outages, dangerous floods and high winds that caused millions of dollars in damage.

The U.S. East Coast, home to some of the country's most densely populated cities and costliest waterfront real estate, was expected to suffer a multibillion-dollar disaster, experts forecast.

In the Carolinas, swells and waves up to 9 feet (3 meters) were reported along the Outer Banks and thousands had already lost power as the fringes of the storm began raking the shore.

In addition to widespread wind and water damage, Irene could also push crude oil prices higher if it disrupts refineries in Delaware, New Jersey, Pennsylvania and Virginia, which produce nearly 8 percent of U.S. gasoline and diesel fuel.

The center of the storm was still about 265 miles (427 kilometers) south-southwest of Cape Hatteras, North Carolina, and moving to the north at 14 mph (22 kph). Forecasters warned wind-whipped water could create a dangerous storm surge, with levels along the state's Albemarle and Pamlico sounds rising as much as 11 feet (3.35 meters).

In Washington, Irene dashed hopes of dedicating a 30-foot (10-meter) sculpture to Martin Luther King Jr. on Sunday on the National Mall. While a direct strike on the nation's capital appeared slim, organizers said the forecasts of wind and heavy rain made it too dangerous to summon a throng they expected to number up to 250,000.

Hundreds of thousands of New Yorkers were told Thursday to pack a bag and be prepared to move elsewhere. Gov. Andrew Cuomo said public transportation in New York City would shut down around noon (1700 GMT) Saturday, and major bridges also could shut down if conditions become too windy.

The nation's biggest city has not seen a hurricane in decades, and a hurricane warning hasn't been issued there since Hurricane Gloria hit in 1985 as a Category 2 storm, said Ashley Sears, a meteorologist with the National Weather Service.

Even if the winds aren't strong enough to damage buildings in a metropolis made largely of brick, concrete and steel, a lot of New York's subways and other infrastructure are underground, making them subject to flooding.

New York's two airports are close to the water and could be inundated, as could densely packed neighborhoods, if the storm pushes ocean water into the city's waterways, officials said.

The five main New York City-area airports will be closed to arriving passenger flights beginning at noon (1700 GMT) on Saturday, aviation officials said.

The Port Authority of New York and New Jersey, which operates the airports and area bridges and tunnels, said Friday that many weekend departures already had been canceled in anticipation of the hurricane.

Hundreds of flights could be affected, Port Authority spokesman Steve Coleman said. The airports handle about 3,000 flights each day on average during the week, fewer on weekends, he said.

In 2008, the city had a brush with Tropical Storm Hanna, which dumped 3 inches (8 centimeters) of rain on Manhattan.

In the last 200 years, New York has seen only a few significant hurricanes. In September 1821, a hurricane raised tides by 13 feet (4 meters) in an hour and flooded all of Manhattan south of Canal Street, the southernmost tip of the city. The area now includes Wall Street and the World Trade Center memorial.

An infamous 1938 storm dubbed the Long Island Express came ashore about 75 miles (120 kilometers) east of the city and then hit New England, killing 700 people and leaving 63,000 homeless.

The first U.S. injuries from Irene appeared to be in South Florida near West Palm Beach, where eight people were washed off a jetty Thursday by a large wave churned up by the storm.

Peltz reported from New York. Associated Press writers Michael Biesecker in Raleigh, North Carolina; Seth Borenstein in New York; Wayne Parry, Geoff Mulvihill and Bruce Shipkowski in New Jersey; Brock Vergakis in Virginia; Randall Chase in Ocean City, Maryland; Harry Weber in Miami; Martha Waggoner in North Carolina; and David Sharp in Portland, Maine, contributed to this story.