Winds of more than 80mph winds and a record-breaking 13ft surge of seawater hit New York City, flooding tunnels, highways, subway stations and the electrical system that powers Wall Street.
Much of the city was deserted, as its subways, buses, commuter trains, bridges and airports were closed.
At least 6.2million people across eastern America were left without power and New York's main utility company said large sections of lower Manhattan had been plunged into darkness by the storm.
The electricity crisis meant more than 200 people had to be moved from New York University Hospital as its back-up generators also failed.
President Obama has declared a major disaster in New York and Long Island. The decision makes federal funding available to people in the area which bore the brunt of the sea surge from the storm.
Storm damage has been projected at $10billion to $20bn (£6.22bn-£12.44bn), meaning it could prove to be one of the costliest natural disasters in US history.
More than a million people in a dozen states were under orders to evacuate as the storm ploughed westward.
A fire destroyed at least 50 homes in a flooded neighbourhood in the New York City borough of Queens. More than 190 firefighters were tackling the blaze.
Three towns in New Jersey, just west of New York, were inundated with up to 5ft of water after a levee on the nearby Hackensack River was breached,
The superstorm also forced President Barack Obama and Republican nominee Mitt Romney to cancel campaign events in key battleground states. That hit both campaigns' carefully mapped out strategies to make their closing arguments to voters with only a week left before the election.
The New York Stock Exchange was closed again today – the first time it has been shut for two consecutive days due to weather since 1888, when a blizzard struck the city.
The National Hurricane Centre said the first area Sandy touched when it came ashore was near Atlantic City.
It hit the boarded-up big cities of the Northeast corridor, from Washington and Baltimore to Philadelphia, New York and Boston.
Sixteen deaths were reported in New Jersey, New York, Maryland, North Carolina, West Virginia, Pennsylvania and Connecticut. Some of the victims were killed by falling trees.
Among those killed in New York City was a woman who stepped into an electrified puddle of water.
Canada was also affected.Police in Toronto said a woman was killed by a falling sign as high winds closed in on the country's largest city.
Sandy has been labelled a superstorm because, as it made its way towards land, it converged with a cold-weather system, becoming a monstrous hybrid consisting not only of rain and high wind but of snow.
Forecasters warned 20ft waves would hit the Chicago lakefront and said there would be up to 3ft of snow in West Virginia.
Before hitting land the storm sunk the replica tall ship HMS Bounty, killing one crew member and leaving the captain missing, presumed drowned.
Just before its centre reached land, Sandy was stripped of its hurricane status, but the distinction was purely technical, based on its shape and internal temperature.
It still packed hurricane-force winds, and forecasters were careful to say it remained every bit as dangerous to the 50million people in its path.
The storm is said to be about 1000 miles wide.
Heavy rain and further flooding remain major threats over the next couple of days.
Today, the centre of the storm was just outside Philadelphia.
"We knew this was going to be a very dangerous storm, and the storm has met our expectations," said New York Mayor Michael Bloomberg said. "This is a once-in-a-long-time storm."
Sandy killed 66 people in the Caribbean last week.