US President Barack Obama has declared a "major disaster" in New York state after storm Sandy smashed into the US East Coast, causing flooding and cutting power to millions.
A record 4m (13ft) tidal surge sent seawater cascading into large parts of New York City's subway system.
Across the city, a power sub-station exploded, a hospital was evacuated and fire destroyed 50 homes.
At least 15 people are reported dead across several US states.
They include:
At least five people killed by the storm in New York state, including a 30-year-old man crushed by a falling tree in the New York City borough of Queens
Three killed in New Jersey, including two parents whose car was crushed by a falling tree; their two children, 11 and 14, survived
A woman in West Virginia who died in a collision in heavy snow caused by the storm
In northern New Jersey, a rescue operation is reported to be under way after a levee broke and flooded the town of Moonachie.
Bergen County chief of staff Jeanne Baratta told CNN: "Moonachie has been devastated. Every street has got four or five feet of water on it."
At least six million homes and businesses are without power. In all, about 50 million people could be affected by the storm, with up to a million ordered to evacuate their homes.
The Battery Tunnel in New York City was flooded
Sandy, now downgraded from a hurricane but described as a "super-storm", is churning north and heading for Canada.
Over the past week, Sandy has killed more than 80 people as it carved a path of destruction through the Caribbean.
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The New York City subway system is 108 years old, but it has never faced a disaster as devastating as what we experienced last night”
New York City transport director Joseph Lhota
Storm Sandy: Your stories
In pictures: After Storm Sandy
The storm made landfall close to Atlantic City in New Jersey on Monday at about 20:00 local time (midnight GMT), with winds of more than 80mph (129km/h).
It collided with cold weather fronts from the west and north to create what some forecasters have dubbed a "Frankenstorm".
Much of Atlantic City was under water and 30,000 residents were evacuated.
In New York City, parts of Lower Manhattan were quickly inundated as the Hudson and East rivers overflowed. Seawater poured into road tunnels and the subway system. Images showed cars being swept along streets by the torrent.
"The New York City subway system is 108 years old, but it has never faced a disaster as devastating as what we experienced last night," city transport director Joseph Lhota said early on Tuesday.
City officials had earlier ordered some 375,000 residents out of Lower Manhattan and other areas under threat.
"Lower Manhattan is being covered by seawater," Howard Glaser, director of operations for the New York state government, was quoted as saying. "I am not exaggerating. Seawater is rushing into the Battery Tunnel."
Battery Tunnel links Manhattan with Long Island.
The city's Consolidated Edison utility provider said an explosion at a sub-station, probably caused by flooding or flying debris, blacked out much of Lower Manhattan.
Matthew Trowbridge and Susan Burke Mangano filmed flooding in Ocean City, New Jersey
The company said about 500,000 homes in Manhattan were without power.
As dawn broke, residents emerged to see the havoc wreaked by the storm.
In other developments:
Fire has destroyed about 50 homes in the New York City borough of Queens
More than 200 patients were evacuated from New York University's Tisch Hospital after power went out and a backup generator failed
A large tanker ship has been washed on to a street in Staten Island, New York
America's oldest nuclear power plant, Oyster Creek in New Jersey, was put on alert due to rising water, the Nuclear Regulatory Commission said
The New York Stock Exchange will stay shut on Tuesday - the first time it has closed for two consecutive days owing to weather since 1888
A crew member from a replica of HMS Bounty has died and the captain is missing after the ship sank in mountainous seas off North Carolina on Monday
A construction crane in New York was bent double next to a skyscraper, while the facade of a four-storey building collapsed.
New York Mayor Michael Bloomberg said the storm surge had surpassed the highest forecast, but he expected waters to start receding.
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SANDY HITS EAST COAST
At least 16 people dead, including one in Canada
1m ordered to leave their homes
50m people estimated to be affected, with 6m left without power
800-mile (1,290 km) stretch of the US affected
10,000 flights reported grounded globally on Monday and Tuesday
Manhattan hit by record storm surge of 13.7ft (4.15m) on Tuesday morning
At 05:00 EDT (09:00 GMT) the National Hurricane Center placed the centre of Sandy about 90 miles (145km) west of Philadelphia with maximum sustained winds of 65mph (105km/h) with higher gusts.
Forecasters have said Sandy could linger over as many as 12 states for 24-36 hours.
President Obama has also declared emergencies in Massachusetts, Connecticut, Rhode Island, New Jersey and Pennsylvania.
Public transport has been suspended in the US capital, New York, Baltimore, Philadelphia and Boston.
Amtrak has suspended passenger train services across the north-east, while nearly 14,000 flights were cancelled, according to Flightaware.com.
Up to 3ft (91cm) of snow was expected to fall on the Appalachian mountains in West Virginia, Virginia and Kentucky.
The disaster-estimating firm Eqecat has forecast that Sandy could cause economic losses to the US of between $10bn and $20bn (£6.2bn-£12.4bn)