NEW YORK - New York City ordered its first limited evacuations Thursday as Hurricane Irene swept north, on track to be the first hurricane to blast the Big Apple in a generation.

Mayor Michael Bloomberg ordered the evacuation of hospitals and other vulnerable institutions in the most low-lying areas of New York City, as New York state declared a state of emergency.

"Seniors homes and nursing homes and these hospitals must evacuate" unless exempted on a case-by-case basis, Bloomberg said, adding in a televised press conference that the evacuations were to be completed by Friday.

In neighboring New Jersey, authorities in the remote Cape May area ordered mandatory evacuation for up to 750,000 people.

New York City officials were to decide by early Saturday whether to order mandatory evacuations of other residents -- up to quarter of a million people -- in high-risk areas, which include stretches of coastline in Brooklyn, the tip of Manhattan and large parts of Staten Island.

"It is very conceivable that at Saturday morning... we're going to say 'you must get out of these areas,'" Bloomberg said.

Another disruption looming over the city was the likely shutting down of mass transit once high winds strike late Saturday through to Monday.

"You're fundamentally not going to have mass transit to take you around from some time Saturday afternoon," Bloomberg said.

City services, including the transportation department, were already moving equipment out of potential flood areas and police were deploying boats around the city.

New York has not faced a hurricane for a quarter of a century and its airports, busy road traffic, maze of underground trains and densely packed waterside communities could make it especially vulnerable.

With city residents still shaken by a rare earthquake Tuesday -- causing no damage, but wobbling skyscrapers -- the impending storm is causing alarm.

Forecasters say the fate of New York, as well as other big cities in the hurricane's sights like Philadelphia, depends on the exact path that Irene takes.

If the system tracks a little more out to sea, that may save the cities a pasting, but if Irene's eye homes in on the coastline, as it currently appears set to do, the Big Apple itself could be in trouble.

The biggest potential danger will come from flooding and falling trees when the rain pours into ground already soaked by weeks of wet weather.

In addition to the downpour, huge surges of wind-driven seawater are expected on top of new moon high tides.

Bloomberg reassured residents that for now there was no fear of damage to Manhattan's skyscrapers.

"Big buildings are designed to withstand enormous winds," Bloomberg said. "There's no reason for you to leave a big building."

Rather, the mayor said, there is likely to be damage in "very old, rather low rise wooden buildings, brick buildings."

"You're going to see small bricks come down," he said.

Among the other unknowns for now is whether the weather will affect outdoor events in New York, including the US Open tennis tournament at Flushing Meadows, Bloomberg said.

New York has not suffered a direct hit from a hurricane since Gloria in September 1985, which hit Long Island as a category one storm causing widespread damage.

New Jersey evacuations

Residents in an isolated coastal community in New Jersey were ordered Thursday to leave their homes ahead of the expected arrival of Hurricane Irene.

The local government order went into effect Thursday for people in barrier islands off Cape May and for all other people in the county, including those inland, starting early Friday.

The order was broadcast in local media and on the official website.

Officials said the entire population could amount to about 760,000 people, the Cape May County Herald reported.

The county's emergencies director, Frank McCall, was quoted as asking people who refused to leave to put an identity card in their left shoe "so they may be identified."

In addition to more than a foot (30 centimeters) of rain, officials expected a storm surge from the Atlantic to add to an already big new Moon tide, adding to the risk of severe flooding.

Irene is set to slam into North Carolina early Saturday before roaring toward New York City, forecasters have said.