Authorities have ordered the evacuation of 375,000 people on the east coast of the United States ahead of the arrival of Hurricane Sandy.
New York mayor Michael Bloomberg launched the mandatory evacuation and said 72 centres had been set up around the city in schools and community centres.
The full public transport system has also been shut down and the New York Stock Exchange will only continue with electronic transactions.
"This is a serious and dangerous storm," Mr Bloomberg said.
"If you don't evacuate you are not just putting your own life in danger, you are also endangering the lives of the first responders who may have to come in and rescue you."
The New York Stock Exchange announced it would close its trading floor but continue to trade electronically, despite fears from some experts that flooding could knock out the vital underground network of power, phone and high-speed Internet lines.
Officials also postponed today's reopening of the Statue of Liberty, which had been closed for a year for \$30m (£22m) in renovations.
The storm is expected to start hitting the area shortly, with the worst weather continuing into Tuesday.
Experts estimate it will affect up to 60 million people in the area, when it meets a winter storm and cold front. The storm surge will be boosted by storm tides from a full moon.
Parts of West Virginia, Virginia, Kentucky and North Carolina could get 2ft or more of snow in places, meteorologists said.
Experts said the rare hybrid storm will cause havoc over 800 miles, from the Atlantic coast inland to the Great Lakes.
British Airways flights to and from New York's JFK airport, Newark airport in New Jersey, Baltimore airport in Maryland and Boston airport in Massachusetts, have been cancelled.
Virgin Atlantic flights to and from New York, Washington and Boston have also been halted.
British Airways said : "We understand that customers may be disappointed, however their safety is our highest priority. We are offering the option to rebook or receive a refund to those customers whose flights are cancelled."
Virgin Atlantic warned passengers of cancelled flights not to travel to departure airports and advised travellers to check their flight status page for the latest information.
Domestic airlines also moved planes out of airports to avoid damage, and added Sunday flights out of New York and Washington ahead of today's flight cancellations.
All of New York City's state schools have been closed, and the evacuation zone includes parts of Coney Island, Manhattan Beach and other areas along the east river in Brooklyn.
Stretches of the lower east side, Staten Island and Manhattan are also included in the danger zone and President Barack Obama said authorities needed to take Sandy "seriously" and advised residents to listen to state officials for guidance.
"We don't yet know where it's going to hit, where we're going to see the biggest impacts. And that's exactly why it's so important for us to respond big and respond fast as local information starts coming in," Mr Obama said.
"My main message to everybody involved is that we have to take this seriously.
"The federal government is working effectively with the state and local governments. It’s going to be very important that populations in all the impacted states take this seriously, listen to your state and local elected officials."
US rail operator Amtrak began cancelling train services on Saturday night, including services between Washington and New York.
Hurricane Sandy is heading north from the Caribbean, where it has killed 65 people.
The majority of the deaths happened in Haiti and the area around the capital Port-au-Prince, which holds most of the 370,000 Haitians who are still living in flimsy shelters as a result of the devastating 2010 earthquake.
Officials in Haiti said 51 people have died there although the number is expected to rise.
The US National Hurricane Centre said that the storm has top sustained winds of 75 mph, with higher gusts. It is moving toward the northeast at 14mph, with hurricane-force winds extend up to 175 miles from the epicentre.