New York City's transit system was forced to close and traders in London were left in limbo today after Hurricane Sandy forced Wall Street to shut down for the first time since the September 11 attacks.
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Dozens of flights to and from the east coast of America have also been cancelled.
British Airways has grounded all flights to and from the US Eastern Seaboard, including New York, Baltimore, Washington, Boston and Philadelphia.
A number of BA flights from the US to London have also been halted.
Virgin Atlantic has cancelled flights to and from New York, Washington and Boston.
Investors struggled to find direction after it was confirmed that America's two biggest trading platforms - the New York Stock Exchange and the Nasdaq Stock Market - will not open as scheduled at 1.30pm GMT.
The markets - home to some of the world's biggest companies including Apple, Microsoft and Starbucks - also look set to be closed tomorrow, while commentators fear the suspension could run into Wednesday.
New York Governor Andrew Cuomo announced that train, subway and bus services would close at 7pm local time (11 pm GMT).
Michael Bloomberg, the city's Mayor, ordered the evacuation of low-lying areas, including parts of Manhattan. Up to 375,000 people may have to leave their homes. Schools will be also shut. "This is a serious and dangerous storm," Mr Bloomberg said.
The hurricane is threatening 50 million people on America's heavily-populated East Coast today and forecasters warned that New York could bear the brunt of the one-of-a-kind superstorm.
Federal Emergency Management Administrator Craig Fugate warned that the "time for preparing and talking is about over" as Sandy made its way up the Atlantic on a collision course with two other weather systems that could turn it into one of the most fearsome storms on record in the US.
"People need to be acting now," he said.
Forecasters warned the megastorm could wreak havoc over 800 miles from the East Coast to the Great Lakes. States of emergency were declared from North Carolina to Connecticut.
As rain from the leading edges of the monster hurricane began to fall over the Northeast, tens of thousands of people in coastal areas from Maryland to Connecticut were under orders to clear out. That included 50,000 in Delaware alone and 30,000 in Atlantic City, New Jersey, where the city's 12 casinos were forced to shut down for only the fourth time in the 34-year history of legalised gambling there.
Authorities warned that the biggest US city could get hit with an 11ft wall of water 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 centre.
Sandy, a Category 1 hurricane, was about 470 miles south east of New York City and the centre of the storm is expected to be near the mid-Atlantic coast tonight.
The National Hurricane Centre said today that the storm had top sustained winds of 75mph, with higher gusts. It is moving towards the north east at 14mph. Hurricane-force winds extend up to 175 miles from the storm's centre.
Sandy was expected to hook left towards the mid-Atlantic coast and come ashore late today or early tomorrow, most likely in New Jersey, colliding with a wintry storm moving in from the west and cold air streaming down from the Arctic.
Forecasters said the monster combination could bring close to a foot of rain, a potentially lethal storm surge and punishing winds extending hundreds of miles outward from the storm's centre. It could also dump up to 2ft of snow in Kentucky, North Carolina and West Virginia.
Louis Uccellini, environmental prediction chief for the National Oceanic and Atmospheric Administration, said given Sandy's east-to-west track into New Jersey, the worst of the storm surge could be just to the north, in New York City, Long Island and northern New Jersey.
Forecasters said that because of giant waves and high tides made worse by a full moon, the metropolitan area of about 20 million people could be slammed by an 11ft wall of water.
"This is the worst-case scenario," Mr Uccellini said.
New York mayor Michael Bloomberg warned people in low-lying areas of lower Manhattan and Queens to get out.
"If you don't evacuate, you are not only endangering your life, you are also endangering the lives of the first responders who are going in to rescue you," he said. "This is a serious and dangerous storm."
New Jersey's famously blunt governor Chris Christie was less polite. "Don't be stupid. Get out," he said.
Officials also postponed today's reopening of the Statue of Liberty, which had been closed for a year for 30 million dollars of renovation.
In Washington, President Barack Obama promised the government would "respond big and respond fast" after the storm hit.
"My message to the governors as well as to the mayors is anything they need, we will be there, and we will cut through red tape. We are not going to get bogged down with a lot of rules," he said.
The storm forced the president and Republican rival Mitt Romney to rearrange their campaign schedules in the crucial closing days of the election race and early voting today in Washington and Maryland was cancelled.
Shelters across the region began taking in people.
"We were told to get the heck out. I was going to stay, but it's better to be safe than sorry," said Hugh Phillips, one of the first in line when a Red Cross shelter in Lewes, Delaware, opened at noon.
"I think this one's going to do us in," said Mark Palazzolo, who boarded up his bait-and-tackle shop in Point Pleasant Beach, New Jersey, with the same wood he used in past storms, crossing out the names of Hurricanes Isaac and Irene and spray-painting "Sandy" next to them.
At least twice as many train passengers as usual crowded the Amtrak waiting area at New York's Penn Station yesterday.