NEW YORK — From Washington to Boston, big cities and small towns buttoned up on Sunday against the onslaught of a superstorm that could menace 50 million people in the most heavily populated corridor in the nation, with forecasters warning New York could be in particular peril.

“The time for preparing and talking is about over,” Federal Emergency Management Administrator Craig Fugate said as Hurricane Sandy made its way up the Atlantic on a collision course with two other weather systems that could turn it into one of the most fearsome storms on record in the U.S. “People need to be acting now.”

Airlines canceled more than 5,000 flights and Amtrak began suspending train service across the Northeast. New York and Philadelphia moved to shut down their subways, buses and trains Sunday night and announced that schools would be closed today. Boston, Washington and Baltimore also called off school.

As rain from the leading edges of the monster hurricane began to fall over the Northeast, hundreds of thousands of people in coastal areas from Maryland to Connecticut were under evacuation orders Sunday, including 375,000 in lower Manhattan and other parts of New York City, 50,000 in Delaware and 30,000 in Atlantic City, N.J., where the city’s 12 casinos were forced to shut down for only the fourth time ever.

“We were told to get the heck out. I was going to stay, but it’s better to be safe than sorry,” said Hugh Phillips, who was one of the first in line when a Red Cross shelter in Lewes, Del., opened at noon.

“I think this one’s going to do us in,” said Mark Palazzolo, who boarded up his bait-and-tackle shop in Point Pleasant Beach, N.J., with the same wood he used in past storms, crossing out the names of Hurricanes Isaac and Irene and spray-painting “Sandy” next to them. “I got a call from a friend of mine from Florida last night who said, ‘Mark. Get out! If it’s not the storm, it’ll be the aftermath. People are going to be fighting in the streets over gasoline and food.’”

Sandy, a Category 1 hurricane with sustained winds of 75 mph as of Sunday evening, was blamed for 65 deaths in the Caribbean before it began churning up the Eastern Seaboard. As of 5 p.m., it was centered about 530 miles southeast of New York City, moving at 15 mph, with hurricane-force winds extending an incredible 175 miles from its center.

It was expected to hook left toward the mid-Atlantic coast and come ashore late today 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 said the combination could bring close to a foot of rain, a potentially lethal storm surge and punishing winds that could cause widespread power outages that last for days. The storm could also dump up to 2 feet of snow in Kentucky, North Carolina and West Virginia.

Louis Uccellini, environmental prediction chief for the National Oceanic and Atmospheric Administration, told The Associated Press that given Sandy’s east-to-west track into New Jersey, the worst of the storm surge could be just to the north, in New York City, on Long Island and in northern New Jersey.

Forecasters said that because of giant waves and high tides made worse by a full moon, the metropolitan area of about 20 million people could get slammed with an 11-foot wall of water.

“This is the worst-case scenario,” Uccellini said.

New York Mayor Michael Bloomberg warned: “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. This is a serious and dangerous storm.”

New Jersey’s famously blunt Gov. Chris Christie was less polite: “Don’t be stupid. Get out.”

New York called off school today for the city’s 1.1 million students and announced it would suspend all train, bus and subway service Sunday night, in part because of the risk of flooding in the subway tunnels. More than five riders a day depend on the transit system.

The New York Stock Exchange announced it will shut down its trading floor today but continue to trade electronically, despite fears from some experts that flooding could knock out the underground network of power, phone and high-speed Internet lines that are vital to the nation’s financial capital.

Officials also postponed today’s reopening of the Statue of Liberty, which had been closed for a year for $30 million in renovations.

In Washington, President Barack Obama promised the government would “respond big and respond fast” after the storm hits.

“My message to the governors as well as to the mayors is anything they need, we will be there, and we will cut through red tape. We are not going to get bogged down with a lot of rules,” he said.

He also pleaded for neighborliness: “In times like this, one of the things that Americans do is we pull together and we help out one another And so, there may be elderly populations in your area. Check on your neighbor, check on your friend. Make sure that they are prepared. If we do, then we’re going to get through this storm just fine.”

The storm forced the president and Mitt Romney to rearrange their campaign schedules in the crucial closing days of the presidential race. And early voting today in Maryland was canceled.

Despite the dire warnings, some souls were refusing to budge.

Jonas Clark of Manchester Township, N.J. — right in the area where Sandy was projected to come ashore — stood outside a convenience store, calmly sipping a coffee and wondering why people were working themselves “into a tizzy.”

“I’ve seen a lot of major storms in my time, and there’s nothing you can do but take reasonable precautions and ride out things the best you can,” said Clark, 73. “Nature’s going to what it’s going to do. It’s great that there’s so much information out there about what you can do to protect yourself and your home, but it all boils down basically to ‘use your common sense.”’

In New Jersey, Denise Faulkner and her boyfriend showed up at the Atlantic City Convention Center with her 7-month-old daughter and two sons, ages 3 and 12, thinking there was a shelter there. She was dismayed to learn that it was just a gathering point for buses to somewhere else. Last year, they were out of their home for two days because of Hurricane Irene.

“I’m real overwhelmed,” she said as baby Zahiriah, wrapped in a pink blanket with embroidered elephants, slept in a car seat. “We’re at it again. Last year we had to do it. This year we have to do it. And you have to be around all sorts of people — strangers. It’s a bit much.”

Before leaving their home in Atlantic City, John and Robshima Williams of packed their kids’ Halloween costumes so they could go bunk-to-bunk trick-or-treating at a shelter. Her 8-year-old twins are going as the Grim Reaper and a zombie, while her 6-year-old plans to dress as a witch.

“We’re just trying to make a bad situation good,” the mother said. “We’re going to make it fun no matter where we are.”