NEW YORK (AP) -- New York City shut down its mass transit system, closed its schools and ordered hundreds of thousands of people to flee their homes Sunday in the face of increasingly dire predictions about the wall of water that could hit the nation's largest city as part of the superstorm bearing down on the East Coast.
A seawater surge of anywhere from 6 to 11 feet threatened to swamp parts of lower Manhattan, flood subway tunnels and knock out the underground network of power, phone and high-speed Internet lines that are the lifeblood of America's financial capital.
Subway, bus, Metro-North and Long Island Rail Road service were all suspended Sunday night. Officials said bridge and tunnel closings would be decided on a "case-by-case basis."
While the New York Stock Exchange will close its trading floor, Big Board trading will continue electronically.
The United Nations announced plans to close on Monday and canceled all meetings at its headquarters.
After days of seeming to take the storm in stride, New Yorkers packed grocery stores for supplies and scrambled to get out of flood zones.
Clutching a white pillow in her left hand and two computers in another, Alyssa Marks rushed to get to the subway before it stopped running Sunday evening.
As she hurried to leave her apartment in a lower Manhattan evacuation zone for a friend's place on higher ground, she'd gotten cash but had not time to get toiletries and water.
"I'm nervous, but I'm also excited," she said as she left her apartment. "We're going to get some wine, cuddle up and watch movies."
Still, some hardy residents said they weren't going anywhere, even as the mayor urged them to go.
"If you don't evacuate, you're not just putting your own life in danger — you are also endangering the lives of our first responders who may have to come in and rescue you," Mayor Michael Bloomberg said at a news conference Sunday as he announced a mandatory evacuation affecting 375,000 people in low-lying areas from the beaches of Queens to the glassy high-rises of Battery Park City. "This is a serious and dangerous storm."
After days of more modest warnings, the tone grew sharper Sunday as the National Hurricane Center predicted "life-threatening flooding" for areas including New York Harbor. Louis Uccellini, the National Oceanic and Atmospheric Administration's environmental protection chief, called the projected storm surge "the worst-case scenario" for New York City, Long Island and northern New Jersey.
Gov. Andrew Cuomo deployed National Guard troops to the city and Long Island as President Barack Obama signed an emergency declaration enabling the federal government to provide support and resources to state and local governments.
Consolidated Edison weighed the possibility of shutting down power in parts of lower Manhattan to protect equipment. Broadway shows were canceled for Sunday and Monday. One small hospital was being evacuated, while several others were moving patients to higher floors.
America's biggest public school system, which serves 1.1 million students, was ordered closed Monday, while many of the schools opened Sunday as emergency shelters.
Ralph Gorham watched the sea get rough, but he planned to weather the storm at the Red Hook Lobster Pound, the seafood business he co-owns in a low-lying part of Brooklyn.
"I'm not leaving. My house is here. My business is here," he said. "When the bell tolls, you live with it."
The megastorm — a predicted combination of Hurricane Sandy, a wintry system moving in from the west and cold air streaming down from the Arctic — marked the second time in 14 months that New York City has faced a scenario forecasters have long feared: a big hurricane hitting the city or a bit south, such that the cyclone's counterclockwise winds drive water into miles of densely populated shoreline.
Hurricane Irene ultimately came ashore as a tropical storm in Coney Island, with a four-foot storm surge that washed over parts of the southern tip of Manhattan but didn't wreak the havoc that officials had feared, although it caused tremendous damage elsewhere. Some experts have said that a surge three feet higher could have caused huge damage.
The deluge from the approaching superstorm may well be worse than Irene's — and longer lasting, forecasters said. Bloomberg warned that damaging flooding could start with high tide Sunday night and continue into Tuesday afternoon.
With the worst of the surge expected Monday night, the key factor is whether it coincides with the higher-than-usual full moon high tide, said Klaus Jacob, a Columbia University researcher who has advised the city on coastal risks. But in any case, he said he expected the subway system to be at least partially flooded.
"I do not think that there's enough emergency measures that will help prevent the subway from flooding," Jacob said.
Bloomberg announced evacuations at about 11:30 a.m., telling people to be out just 7½ hours later. The city was alerting residents with cellphone emergency warning systems, its own Notify NYC system of emails and phone calls, and police cars going down streets with bullhorns.
To those who refused to leave, Bloomberg had a message.
"They won't be arrested. But I would argue they are being very selfish," said the mayor, noting rescue crews will still try to help them if they are flooded. "We aren't going to leave them to die. We are going to save them."
Associated Press writers Verena Dobnik and Deepti Hajela in New York and Seth Borenstein in Kensington, Md., contributed to this report.