Ferocious winds from Hurricane Irene have begun to hammer New York, bringing torrential rain and the threat of flooding in the financial district.
New York City's public transport system has been closed and the mayor said it was now too late for people to leave.
Irene has already hit North Carolina and Virginia, causing damage and the deaths of at least eight people.
The eye of the storm, packing winds up to 75mph (120km/h), is due to hit New York in the next few hours.
The storm has weakened to a category-one hurricane since it came ashore but is still expected to be destructive.
About two million people have been left without power as the 500-mile-wide (800km) storm barrelled up the east coast.
The same number have moved out of the danger zone, most from New Jersey.
At 05:00 (09:00 GMT) the hurricane was moving along the New Jersey shore, about 115 miles south of New York City, having weakened slightly, the National Hurricane Center reported.
Some 370,000 people living in low-lying areas of New York City had been told to leave, but Mayor Michael Bloomberg warned those behind to stay put.
"The edge of the hurricane is finally upon us," he told reporters.
"The time for evacuations is over. At this point, if you haven't evacuated, our suggestion is you stay where you are.
"Nature is a lot stronger than the rest of us."
The fear is of a storm surge affecting New York's Hudson River, which could potentially inundate the flood defences of Lower Manhattan and cause flooding in the financial district there.
On Wall Street, sandbags have been placed around subway grates and construction work has been stopped across the city.
Irene has already dumped more than 1ft (30cm) of rain on North Carolina and Virginia and there are reports of storm surges of nearly 10ft.
The north-eastern seaboard is the most densely populated corridor in the US, with more than 65 million people living in major cities from Washington DC in the south to Baltimore, Philadelphia, New York and Boston further north.
Scott Snyder of the American Red Cross told the BBC about 13,000 people have taken refuge in 150 Red Cross shelters along the coast.
States of emergency have been declared in North Carolina, Maryland, Virginia, Delaware, New Jersey, New York and Connecticut. New York's John F Kennedy and LaGuardia airports, and Newark in New Jersey, have shut, with the cancellation of about 8,000 flights.
The hurricance has been blamed for the deaths of two children, as well as six other people killed by falling trees, road accidents and high waves, in North Carolina, Virginia, Maryland and Florida.
A nuclear reactor in Maryland automatically went offline after a large piece of aluminium blown down by high winds came into contact with the plant's main transformer, prompting a low-level emergency.
Echoes of Katrina
President Barack Obama cut short his holiday to Martha's Vineyard to co-ordinate efforts to deal with the hurricane.
The BBC's David Willis in Washington says the president is very keen to avoid any criticism that surrounded the federal government's response to Hurricane Katrina six years ago.
The president is at pains to point out that all the federal agencies on the ground that need to be deployed have been and that he is looking to be seen to be on top of this and will be over the next few hours and days, our correspondent says.
The Pentagon has loaded 200 trucks with emergency supplies, and 100,000 National Guard troops are on standby.
Irene made landfall at 08:00 on Saturday at Cape Lookout in North Carolina for what is expected to be a 36-hour assault on the US east coast.
Residents hoping to ride out the storm have stocked up on food, water and fuel.
"Thursday late night I bought bottles of water after New Jersey declared a state of emergency. They were the last bottles of water on the shelves," Jay, from Manhattan, told the BBC.
"[There are] very heavy wind gusts. I live on the 33rd floor so gusts are powerful up here. The rain is coming down consistently hard," he said.
"Last time I checked from my window I only saw police cars on West 34th Street, which never happens. It's one of the busiest streets in Manhattan 24/7."
Supermarkets along the east coast were reportedly running out of supplies before the storm arrived.
"This is my first time witnessing anything like this," student Ryan Narcisse of Roselle, New Jersey, told the BBC. "The street was blanketed with a sheet of water...
"It is tense. It's amazing. The wind. We have to worry about power lines going down which is a major issue. The New Jersey governor has 6,000 electricians ready to fix power lines but I don't think that's going to be enough given the damage that is bound to happen after the storm... We are not used to this at all on the East Coast."