SHIP BOTTOM, N.J. — Shelters opened and tens of thousands of residents were ordered to evacuate coastal areas Sunday as big cities and small towns across the Northeast buttoned up against the onslaught of a superstorm threatening some 50 million people in the most heavily populated corridor in the nation.

“The time for preparing and talking is about over,” Federal Emergency Management Administrator Craig Fugate warned as a monster Hurricane Sandy headed up the Eastern Seaboard on a collision course with two other weather systems. “People need to be acting now.”

New York City announced its subways, buses and trains would stop running Sunday night because of the danger of flooding, and its 1.1 million-student school system would be closed on Monday. Mayor Michael Bloomberg also ordered the evacuation of part of lower Manhattan and other low-lying neighborhoods.

“If you don’t evacuate, you are not only endangering your life, you are also endangering the lives of the first responders who are going in to rescue you,” he said. “This is a serious and dangerous storm.”

Tens of thousands of people along the coast in Delaware, New Jersey, Connecticut and other threatened areas were also under orders to clear out because of the danger of as much as a foot of rain, punishing winds of 80 mph or higher and a potentially deadly wall of water 4 to 11 feet high.

Sandy was headed north from the Caribbean, where it left nearly five dozen people dead, and was expected to hook left toward the mid-Atlantic coast and come ashore late Monday or early Tuesday, most likely in New Jersey, colliding with a wintry storm moving in from the west and cold air streaming down from the Arctic.

Forecasters warned that the resulting megastorm could wreak havoc over 800 miles from the East Coast to the Great Lakes. Parts of West Virginia, Virginia, Kentucky and North Carolina could get snow — 2 feet or more in places.

“I’ve been here since 1997, and I never even put my barbecue grill away during a storm,” Russ Linke said shortly before he and his wife left Ship Bottom on Saturday. “But I am taking this one seriously. They say it might hit here. That’s about as serious as it can get.”

He and his wife secured the patio furniture, packed the bicycles into the pickup truck, and headed off the island.

Witlet Maceno, an emergency room nurse working at New York City’s Mount Sinai Hospital, was headed home to Staten Island on Sunday morning after his overnight shift. He said he was going home to check on his parents, visiting from Atlanta, before he returned to work Sunday evening.

“I’m making sure they’re OK, that they have water and food, and that the windows are shut tight,” he said. “And I’m going to remove stuff outside that could go flying into the windows” of his street-level apartment.

The danger was hardly limited to coastal areas, with forecasters worried about inland flooding. They also warned that the rain could saturate the ground, causing trees to topple onto power lines and cause blackouts that could last for several days.

States of emergency were declared from North Carolina, where gusty winds whipped steady rain on Sunday morning, to Connecticut. Delaware ordered 50,000 people in coastal communities to clear out by 8 p.m. Sunday.

Officials in New York City were particularly worried about the possibility of subway flooding. The city closed the subways before Hurricane Irene last year, and a Columbia University study predicted that an Irene surge just 1 foot higher would have paralyzed lower Manhattan.

Sandy was at Category 1 strength, packing 75 mph winds, about 250 miles southeast of Cape Hatteras, N.C., and moving northeast at 14 mph as of 11 a.m. Sunday, according to the National Hurricane Center in Miami. It was about 575 miles south of New York City.

The storm was so big, however, and the convergence of the three storms so rare, that “we just can’t pinpoint who is going to get the worst of it,” said Rick Knabb, director of the National Hurricane Center in Miami.

“You never want to be too naive, but ultimately, it’s not in our hands anyway,” Andrew Ferencsik said as he bought plywood and 2-by-4’s from a Home Depot in Lewes, Del.

Bobbie Foote said she would heed an evacuation order Sunday for south Wilmington, Del., and would take shelter at her daughter’s home in nearby Newark.

“My daughter insists that I leave this time,” said Foote, a 58-year-old fitness coach. It will be the first time she has fled a storm threatening the apartment building that has been her home for at least 40 years in the working-class neighborhood near the Delaware River.

Foote said she stayed last year when flooding from the remnants of Hurricane Irene blocked streets at either end of the neighborhood. She said her daughter wouldn’t stand for her getting trapped that way again.

“She said I should never put myself in that predicament where I cannot get in or out of where I live,” Foote said.

Amtrak began canceling train service Saturday night to parts of the East Coast, including between Washington and New York. Airlines started moving planes out of airports to avoid damage and added Sunday flights out of New York and Washington in preparation for flight cancellations on Monday.

The Virginia National Guard was authorized to call up to 500 troops for debris removal and road-clearing, while homeowners stacked sandbags at their front doors in coastal towns.

In Arlington, just outside Washington, D.C., a few shoppers strolled in and out of a supermarket. Cathy Davis said the supermarket was sold out of the water she wanted to purchase, but she wasn’t doing much else to prepare. She figured she would bring her outdoor furniture inside later in the day, and might make some chili.

She said the storm did lead her to decide against decorating for Halloween.

“I was like, ‘Eh, it will just be blown away anyway,’” she said. “What’s the point?”

President Barack Obama was monitoring the storm and working with state and locals governments to make sure they get the resources needed to prepare, administration officials said.

In North Carolina’s Outer Banks, there was some scattered, minor flooding at daybreak Sunday on the beach road in Nags Head. Rising tides and pounding waves were expected as the day wore on.

DeWitt Quinn of the mainland city of Badin, N.C., was in the Outer Banks for his annual fishing trip when Sandy threatened to disrupt his plans.

“We’ve got cards. We’ve got computers. We’ve got food. We’re going to cook our brains out and eat very well,” Quinn said.

In New Jersey, hundreds of coastal residents started moving inland.

Gov. Chris Christie’s emergency declaration will force the shutdown of Atlantic City’s 12 casinos for only the fourth time in the 34-year history of legalized gambling here. City officials said they would begin evacuating the gambling hub’s 30,000 residents at noon Sunday, busing them to mainland shelters and schools.

The storm also forced the presidential campaign to juggle schedules. Mitt Romney scrapped plans to campaign Sunday in Virginia and switched his schedule for the day to Ohio. First lady Michelle Obama canceled an appearance in New Hampshire for Tuesday, and Obama moved a Monday departure for Florida to Sunday night to beat the storm. He also canceled appearances in northern Virginia on Monday and Colorado on Tuesday.