Hurricane Sandy is closing in on highly populated areas of the US east coast threatening storm surges and devastating flooding.In New York City, thousands of people have been ordered to leave their homes and evacuation shelters have been set up in 76 schools.
Public transport has been halted and the New York Stock Exchange closed.
Forecasters fear Sandy will become a super-storm when it collides with cold weather fronts from the west and north.
Sandy has already killed 60 people after sweeping through the Caribbean in the past week.
Campaigning for the US presidential election has also been disrupted, eight days ahead of election day.
At 05:00 EDT (09:00 GMT), Sandy was churning about 385 miles (615km) south-east of New York City, the National Hurricane Center (NHC) said.Worryingly, forecasters said its maximum sustained winds had increased to 85mph (140km/h) from 75mph recorded hours earlier.
The vast hurricane, about 520 miles (835km) across, is moving slowly north and could linger over as many as 12 states for 24-36 hours, bringing up to 25cm of rain, 60cm of snow, extreme storm surges and power cuts.
The eye of the storm is expected to barrel across the coast of mid-Atlantic states by Monday night, the NHC said.
As it will hit the US East Coast just before Halloween, it is being dubbed a "Frankenstorm".
With emergencies declared in several east-coast states, many workers were staying at home on Monday.
New York City's subway, bus and train services were suspended from Sunday evening, and schools are shut.
Taxi driver Peter Franklin told the BBC that the city was "shut down".
"I feel like I am living in a science fiction movie," he said.
Hundreds of thousands of people from Maryland to Connecticut were ordered to leave low-lying coastal areas.
They included about 375,000 in lower Manhattan and other areas of New York City and another 30,000 in Atlantic City, New Jersey.
Visiting the Federal Emergency Management Agency (Fema) in Washington on Sunday, he vowed the government would "respond big and respond fast" after Sandy had passed.
"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," he said.
Authorities warned that high tides triggered by a full moon could create storm surges of up 11ft (3m), sending seawater surging through parts of lower Manhattan.
The United Nations headquarters in New York also shut down.
New York City Mayor Michael Bloomberg described it as "a serious and dangerous storm."
Addressing those who had been advised to leave, he said: "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."
New Jersey Governor Chris Christie echoed his words, saying: "Don't be stupid. Get out."
Amtrak has suspended passenger train services across the north-east and air travel has been badly hit, with some 6,800 flights cancelled.
The Statue of Liberty was reopened on Sunday after a year of renovation, but only a group of army cadets got a tour before it was shut again until at least Wednesday.
Some 200 National Guardsmen will patrol Manhattan and 300 more will be deployed in Long Island.
Path of Hurricane Sandy