HUNDREDS OF THOUSANDS of New Yorkers were told Thursday to pack a bag and prepare to be evacuated as the nation’s biggest city braced for its first hurricane in decades.

The city’s mayor Michael Bloomberg ordered nursing homes and five hospitals in low-lying areas evacuated beginning Friday and said he would order 270,000 other people moved by Saturday if the storm stays on its current path.

Hurricane Irene was on track to make landfall Saturday in North Carolina and then move up the East Coast, reaching the New York area by late Sunday. “For the general public, it’s a good idea to move Friday,” Bloomberg said. “Keep in mind, it is possible — I don’t know that I want to say likely — but it is very conceivable … that Saturday morning at 8 o’clock, we’re going to say, ‘Look, the forecast has not changed. The storm is still barreling down on us. It’s still very dangerous. You must get out of these areas.’”

Evacuating hundreds of thousands of people would be particularly difficult in New York, where there are about 1.6 million people in Manhattan, many without cars. There are about 6.8 million in the city’s other four boroughs.

“Don’t wait until the last minute,” the mayor said. “If you can move out on Friday, that’s great.”

Bloomberg advised residents on the southern tip of Manhattan and on Brooklyn’s Coney Island to start moving items upstairs and to be ready to leave immediately. Apartment building managers emailed residents, telling them to close windows and expect power outages. Flyers were posted in building lobbies.

“If you have a car and you live in a low-lying area, my suggestion is to park on top of a hill, not in the valley,” Bloomberg said. “It’s those kinds of things. Take some precautions now, so that if it gets to that you’ll have less to do.”

Irene rolled toward the Carolinas on Thursday with winds of 115 mph. The storm was expected to weaken after brushing North Carolina’s Outer Banks, but it will still likely be a hurricane when it rumbles toward the Northeast.

Forecasters said passing near Manhattan could lead to a nightmare scenario: shattered glass falling from skyscrapers, flooded subways and seawater coursing through the streets.

In the last 200 years, New York has seen only a few significant hurricanes. In September of 1821, a hurricane raised tides by 13 feet 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.

Video: NASA space footage captures the scale of Hurricane Irene

Video: The devastation wreaked by the storm in Puerto Rico days ago