Hurricane Sandy picked up strength and turned toward New York City and the East Coast's other largest cities Monday, forcing the shutdown of financial markets and mass transit, sending coastal residents fleeing and threatening high winds, rain and a wall of water up to 11 feet (3.35 meters) tall. It could endanger up to 50 million people for days.

Sandy strengthened before dawn and stayed on a predicted path toward New York, Washington, Baltimore and Philadelphia— putting it on a collision course with two other weather systems that would create a superstorm with the potential for havoc over 800 miles (1,280 kilometers) from the East Coast to the Great Lakes. Up to 3 feet (0.9 meters) of snow were even forecast for mountainous parts of West Virginia.

Airports closed, and authorities warned that the time for evacuation was running out or already past. Many workers planned to stay home as subways, buses and trains shut down across the region under the threat of flooding that could inundate tracks and tunnels. Utilities anticipated widespread power failures.

"Leave immediately. Conditions are deteriorating very rapidly, and the window for you getting out safely is closing," New York Mayor Michael Bloomberg told those in low-lying areas. Water was splashing over the seawalls at the southern tip of Manhattan.

The center of the storm was positioned to come ashore Monday night in New Jersey, meaning the worst of the surge could be in the northern part of that state and in New York City and on Long Island. Higher tides brought by a full moon compounded the threat to the metropolitan area of about 20 million people.

"This is the worst-case scenario," said Louis Uccellini, environmental prediction chief for the National Oceanic and Atmospheric Administration.

Airlines canceled nearly 7,500 flights and Amtrak began suspending train service across the Northeast.

As rain from the leading edges began to fall over the Northeast on Sunday, hundreds of thousands of people from Maryland to Connecticut were ordered to leave low-lying coastal areas, including 375,000 in lower Manhattan and other parts of New York City, 50,000 in Delaware and 30,000 in Atlantic City, New Jersey, where the city's 12 casinos shut down for only the fourth time ever.

President Barack Obama declared emergencies in Massachusetts, Connecticut, Rhode Island, New York, New Jersey and Pennsylvania, authorizing federal relief work to begin well ahead of time. He promised the government would "respond big and respond fast" after the storm hits.

The president made a direct appeal to those in harm's way: "Please listen to what your state and local officials are saying. When they tell you to evacuate, you need to evacuate. Don't delay, don't pause, don't question the instructions that are being given, because this is a powerful storm."

Obama and Republican Mitt Romney called off their campaign events at the height of the presidential race, with just over a week to go before Election Day. Early voting was canceled Monday in Maryland and Washington, D.C.

Authorities warned that New York could get hit with a surge of seawater that could swamp parts of lower Manhattan, flood subway tunnels and cripple the network of electrical and communications lines that are vital to the nation's financial center.

Major U.S. financial markets, including the New York Stock Exchange, Nasdaq and CME Group in Chicago, planned a rare shutdown Monday. The United Nations also shut down.

New York shut down all train, bus and subway service Sunday night. More than 5 million riders a day depend on the transit system. Boston, Philadelphia and Washington, D.C., also shut down their transit systems. Authorities moved to close the Holland Tunnel, which connects New York and New Jersey, and a tunnel between Manhattan and Brooklyn.

Sandy, a Category 1 hurricane with top sustained winds of 90 mph (144 kph) early Monday, was blamed for at least 69 deaths in the Caribbean before it began traveling northward, parallel to the Eastern Seaboard.

The storm was centered about 260 miles (415 kilometers) south-southeast of New York City. As of 11 a.m., it was moving at 18 mph (29 kph), with hurricane-force winds extending an extraordinary 175 miles (281 kilometers) from its center.

Sandy was expected to make landfall Monday evening or night along or just south of the southern New Jersey coast, then collide with a wintry storm moving in from the west and cold air streaming down from the Arctic and cut across into Pennsylvania and travel up through New York state.

New Jersey Gov. Chris Christie was typically blunt: "Don't be stupid. Get out."