As a mammoth storm system conspired to assault the most populous part of the United States, hundreds of thousands of people moved to higher ground because of warnings that the East Coast faces a unique threat of high winds, drenching rainfall and unparalleled floodwaters.

With Hurricane Sandy advancing, the Washington region prepared to shut down as thoroughly as it might under several feet of snow. Federal, state and local government workers were told to stay home on Monday. The entire public transit system — Metro, Virginia Railway Express and the Maryland Transportation System — ceased operations.

One by one, local school systems — including those in the District and Fairfax and Montgomery counties — said they did not plan to open Monday. Some took the rare step of announcing they would not open on Tuesday or Wednesday either. Early voting in the District and Maryland was canceled for Monday.

People were encouraged to stay indoors and off the roads. And Halloween figured to be a washout.

Cities north along the Eastern Seaboard took similar action.

New York Mayor Michael R. Bloomberg (I) ordered the evacuation of Coney Island and Lower Manhattan, and authorities shut down the city’s schools and its subway system, effectively bringing the nation’s largest city to a near halt. More than 60 miles inland, Philadelphia Mayor Michael Nutter (D) urged people to leave low-lying neighborhoods.

Thousands of flights in and out of eastern cities were canceled, and utility crews were summoned from distant states after it was predicted that 10 million people might lose electricity.

In the Washington area, utilities used robo-calls to warn people to prepare to be without power for days or weeks. Some people spent Sunday gathering sandbags and supplies. Coastal resorts in Maryland and Delaware were evacuated.

Weather forecasters and governors spent Sunday warning that Hurricane Sandy and its co-conspirators — a jet stream barricade to the west, a strong nor’easter and a full moon that drives tides to abnormal heights — were not be be trifled with.

“This storm is a killer storm that will likely take more lives as she makes landfall,” said Maryland Gov. Martin O’Malley (D). “This is a very large and unprecedented storm. It will be a couple of days before it will be even safe to get linemen out on the streets [and] up in the bucket trucks and reconnecting people to power.”

Virginia Gov. Robert F. McDonnell (R) said Sandy “is unlike any storm I’ve seen in 20 years in office. It’s a very unique weather event this late in the season for the people of Virginia.

“This is going to be a long haul,” he said. “We will no doubt have rain and high winds through Tuesday and in Northern Virginia significant wind and rains into Wednesday. People are going to have to be patient.”

D.C. Mayor Vincent C. Gray (D) said the storm was “unique, large, dangerous and unlike anything our region has ever experienced in a very a long time.”

In New Jersey, where the core of the storm is anticipated to arrive late Monday or early Tuesday, Gov. Chris Christie (R) warned the most densely populated state in the nation that the storm’s menace was real.

“When the storm comes, if it’s as bad as they’re predicting it will be, you’ll wish you weren’t as cynical,” Christie said. “I’m not trying to be alarmist here. We need to prepare for the worst. I don’t want to see lives lost unnecessarily.”

In Ocean City on the Maryland coast, Sunday dawned as a somber, gray day that presaged the coming storm. With sharp wind, building surf and stinging rain, Sandy delivered a message of intent that met mixed reactions.

“The only reason we’re leaving now is because we have no choice,” Jay Caffman, 50, said as he huddled with his wife on the city’s rain-soaked boardwalk. Evacuation of the lowest-lying areas had forced them to depart.

But Lloyd and Myrtle Pullen of Ocean Pines, just outside Ocean City, said they weren’t planning to leave — at least not yet.

“I’m going to stick around tonight and see what happens,” said Myrtle Pullen, 74.

City Council member Brent Ashley said that people in Ocean City are used to hurricane hype.

“I wait until I see it’s going to be closer until I worry too much,” Ashley said. “Four days out we were right in the bull’s eye, but now it appears to be going more towards New Jersey.”

More than 100 miles to the west in Arlington County, shoppers seemed in no particular rush to get home despite the foreboding skies and grim forecasts.

“We were more worried about [last year’s] Hurricane Irene,” said Claudia Patane, 28, who paused to sample a wine while shopping at the Harris Teeter in Pentagon City. “We’re calmer about this one.”

Greg Lambert also was in no rush. He grabbed a few things at the Bed Bath and Beyond in Pentagon City before heading to the neighboring Harris Teeter to stock up on canned peaches, chips and a ham to cook Sunday night so he has enough food on hand.

“I’ve lived through hurricanes before,” said Lambert, 60, of Crystal City, “and I’ll live through this one.”

Some weren’t bothering to get ready for the storm at all.

“I’m not doing anything any different,” said Ross Milton, 56, who lives near the Pentagon. “I don’t ever prepare for storms.”

Milton said she would have gone to the grocery store on Sunday regardless of the storm.

“I don’t think it’s going to be a big deal for me,” she said. “But if it is, I’ll be sorry.”

Until a storm has a serious impact on her, she said, her plans won’t change. “My husband and I will probably sit on the front porch and watch it,” Milton said.

Brookland Hardware in Northeast Washington offered “Hurricane Sandy specials” Sunday. Residents loaded up with flashlights and sandbags.

“I just hope the storm is not as strong as they say it could be,” Marx Dupree, a customer, said as he waited for a fresh shipment of batteries to arrive.

Weather forecasters were far less sanguine. They said the confluence of intense elements was unlike anything they’d seen before, particularly across a region with almost 60 million people.

“Many have compared Sandy to ‘the Perfect Storm’ of 1991, and experts have warned it may be even worse,” said Jason Samenow of The Washington Post’s Capital Weather Gang. “Generally speaking, though, each storm has its own character, and sometimes you just can’t compare until all is said and done.”

President Obama met with officials at the headquarters of the Federal Emergency Management Agency in Washington on Sunday and spoke on a conference call with governors and mayors in a region from North Carolina to New England and west to Ohio.

“At this stage, everybody is confident that the staging process, the prepositioning of resources, commodities, equipment that are going to be needed to respond to this storm, are in place,” Obama said afterward. “We don’t yet know where it’s going to hit, where we’re going to see the biggest impacts. And that’s exactly why it’s so important for us to respond big and respond fast as local information starts coming in.”

The U.S. Office of Personnel Management said all federal offices in the Washington area would be closed Monday, with all non-emergency employees being granted excused absences for the day. Federal workers who can telework from home — and have electricity to do so — are expected to work Monday.

The Maryland Transit Administration canceled all MARC service and commuter bus operations for Monday.

The Chesapeake Bay Bridge will close if winds top 55 miles per hour.

“We’re assuming anything is possible at this point,” said John C. Sales, a spokesman for the Maryland Transportation Authority.

Scores of flights at Reagan National and Dulles International airports were canceled, and flights at those airports will largely halt Monday morning.

Travelers should not expect flights from 10 a.m. Monday until after the storm passes, said David Mould, a spokesman for the Metropolitan Washington Airports Authority, which operates National and Dulles.

The ripple effect of shutting down flights to most of the East Coast states is expected to snarl national air travel for days after the storm passes.

The first of it was in evidence Sunday at Chicago’s O’Hare International Airport. United Airlines canceled its Sunday afternoon flights to Reagan National, causing Washington-bound passengers to try to get seats on the earlier flights.

“I don’t want to get stranded in Chicago,” Matt Kremer, 40, of Alexandria said when his 3 p.m. flight was canceled. He managed to get on a noon departure.

Amtrak canceled service in the northeast on Monday, shutting down the rail line between Washington and New York.

With utilities from Virginia to Maine asking other companies to loan them line crews, the companies that do business in states that won’t be hit by the storm were stretched thin. There was hope that local power systems rebuilt after the intense windstorm known as the derecho in June might better stand up to Sandy.

“Pepco has committed all its resources to Hurricane Sandy,” said Thomas H. Graham, Pepco’s regional president. “Because of the magnitude of the storm, we will not be issuing estimated restoration times until the storm has passed and a preliminary damage assessment has been conducted. At that time, a global estimated restoration time will be released indicating when we expect to have 90 percent of customers restored.”

Lori Aratani, Jeremy Borden, Emma Brown, Tim Craig, Aaron C. Davis, Hamil R. Harris, Ann E. Marimow, Ed O’Keefe, Colum Lynch and Del Quentin Wilber contributed to this report.