Post-tropical cyclone Sandy is roaring up the US East Coast, threatening major floods and power cuts that could affect 60 million people.
Hundreds of thousands of residents have been ordered to evacuate New York City and the state of New Jersey.
Public transport has been suspended in cities up the eastern seaboard and thousands of flights are grounded.
The storm is poised to make landfall soon near New Jersey. It has already killed 69 people - 52 of them in Haiti.
Sandy lost its hurricane status late on Monday as it neared the coast and collided with winter weather systems.
But forecasters warned that Sandy's hybrid nature would not lessen the life-threatening storm surge it threatens.
At the White House on Monday, President Barack Obama warned Americans in harm's way to follow emergency instructions.
"When they tell you to evacuate, you need to evacuate," said Mr Obama, who has cancelled campaign events eight days before the US elections.
"Do not delay. Don't pause. Don't question the instructions that are being given, because this is a serious storm and it could potentially have fatal consequences."
The president signed nine emergency declarations covering New York, New Jersey, Massachusetts, Connecticut, Rhode Island, Delaware, Pennsylvania, Maryland and District of Columbia.
The storm threatens an 800-mile (1,290-km) swathe of the US, from the Atlantic Ocean to the Great Lakes in the Mid-West.
Up to 3ft (91cm) of snow is expected to fall on the Appalachian mountains in West Virginia, Virginia and Kentucky.
At 19:00 EDT (23:00 GMT), Sandy was churning about 20 miles south of Atlantic City, New Jersey, the National Hurricane Center (NHC) said.
Sandy, dubbed Frankenstorm, is losing some strength as its centre nears land - its maximum winds are near 85mph.
Full moon tides
Hurricane force winds extended for 175 miles and tropical storm force winds for 485 miles, the NHC said earlier.
A storm surge of higher than 12ft was reported on New York's Long Island. Authorities said high tides swelled by a full moon worsened the flood risk.
Forecasters say Sandy could linger over as many as 12 states for 24-36 hours.
President Obama cancelled a planned rally in Florida with former President Bill Clinton on Monday and an event for Tuesday in Wisconsin, far from the storm.
His Republican challenger, Mitt Romney, held two rallies as planned in the Mid-West, but cancelled events later on Monday and Tuesday.
Amtrak has halted passenger train services across the north-east, while nearly 14,000 flights were cancelled, according to Flightaware.com.
In Washington DC, federal government offices were to be closed until Wednesday.
With public transport suspended in the US capital, New York, Baltimore, Philadelphia and Boston, many workers stayed at home.
Authorities have closed some of the major travel arteries across the Hudson River between New York and New Jersey, including the George Washington Bridge and the Holland Tunnel. Maritime operations at shipping terminals in that region were also suspended.
More than one million people have no electricity.
The Hudson and East rivers in New York City have begun overflowing their banks.
Officials warned that flooding in lower Manhattan could inundate the city's underground electric and communications lines and subway system.
Power company Con Edison warned it may have to turn off electric service to customers in a swathe of lower Manhattan.
Trading on the New York Stock Exchange and Nasdaq will stay suspended for a second day on Tuesday. The United Nations headquarters in New York has also been closed.
In Manhattan, a giant construction crane has partially collapsed and is dangling atop a 65-storey skyscraper. No injuries were reported.
Mass evacuations are under way in low-lying coastal areas from Maryland to Connecticut.
Some 375,000 residents were ordered out of lower Manhattan and other areas of New York. The city has set up shelters for the displaced in scores of schools.
Another 50,000 residents were ordered to leave their homes in Delaware.
Some 30,000 were evacuated from Atlantic City, New Jersey, much of which was underwater.
Chincoteague Island on the Virginia coast was reported to be entirely submerged.
The Pentagon has activated 6,700 members of the National Guard and made available 140 helicopters for rescue and relief efforts.
At least 14 out of 16 people on board a replica of HMS Bounty - built for the 1962 film Mutiny on the Bounty - had to be rescued earlier off North Carolina after the ship began taking on water.
A female crew member was taken to hospital after being recovered unresponsive from the water and another remains missing.