"Tonight's the hard night. We're just waiting for it to hit," North Carolina Governor Bev Perdue told CNN.

Washington, D.C. and states from the Carolinas through Maine declared emergencies due to Irene, a nearly 600 mile-wide hurricane that put 55 million Americans on alert and threatened to cause billions of dollars in damage.

"The core of the hurricane will approach the coast of North Carolina tonight and pass near or over the North Carolina coast Saturday morning," the U.S. National Hurricane Center said in an advisory on Friday night.

President Barack Obama said the impact of the unusually large storm could be "extremely dangerous and costly" for a nation that still recalls the destruction of Hurricane Katrina in 2005.

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

At 11 p.m. EDT, Irene's center was 140 miles south of Cape Lookout, North Carolina, and churning north-northeast at 13 miles per hour.

Hundreds of thousands of residents and vacationers were evacuating from Irene's path.

A quarter of a million New Yorkers were ordered to leave homes in low-lying areas, including the financial district surrounding Wall Street in Manhattan, as authorities prepared for a 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 evacuating patients and the mass transit system, which carries 8.5 million people on weekdays, was due to start shutting down around noon on Saturday.