Hurricane Irene hit New York late Saturday, Mayor Michael Bloomberg said as rain and stronger winds began to lash the city.
"The edge of the hurricane is finally upon us," he told a press conference.
Bloomberg said that with mandatory evacuation orders handed to some 370,000 people in low-lying neighborhoods and public transport completely shut down, "the time for evacuations is over."
"At this point, if you haven't evacuated, our suggestion is you stay where you are."
"The bottom line is the storm is now starting to hit New York City," he said. "Nature is a lot stronger than the rest of us."
The hurricane has already killed at least eight people along the US east coast, cut power supplies to nearly a million.
Packing winds of up to 85 miles (140 kilometers) an hour, Irene was a weakened but still deadly category one storm when it made landfall at 8:00 am (1200 GMT) at Cape Lookout, North Carolina, near a chain of barrier islands.
At least eight people, including an 11-year-old boy killed by a falling tree, died in storm-related incidents along the eastern seaboard.
Irene knocked out power supplies for some 900,000 people, triggered the cancellation of more than 8,000 flights, and forced nearly two million people to evacuate.
Bloomberg ordered the unprecedented evacuation of some 370,000 people from flood-prone neighborhoods, while in next-door New Jersey the Governor Chris Christie ordered more than a million people out of their homes.
New York Authorities also shut down the entire Subway and bus system, turning the sprawling city into a ghost town as the leading edge of Irene swept over.
Officials said the biggest danger was to be from flooding caused not just by tropical rainfall but a surge of wind-driven seawater pushing up from the Atlantic.
City areas at risk of being swamped included parts of the financial district in Manhattan and low-lying beach resorts in Brooklyn and Queens nearby on Long Island. Boat owners scrambled to get their craft ashore and officials across New Jersey and New York pleaded with residents to keep off beaches.
Officials say Manhattan's skyscrapers are not at risk of serious damage, but warn that power outages might strand residents without light, water or elevators.
The disruption took on an international character after the area's three big airports -- John F. Kennedy International Airport, LaGuardia and Newark -- were ordered to stop all flights at 10:00pm (0200 GMT).
The flightaware.com website, which tracks airport arrivals and departures, estimated that 8,337 flights would be cancelled during the weekend, mainly US domestic trips. It warned that the figure would rise.
President Barack Obama, who cut short his summer vacation, visited the Federal Emergency Management Agency's operations center in Washington, where he said the east coast was in for a "long 72 hours."
Some 65 million people live in the urban corridor from Washington north to Boston, and experts have said the damage could cost anything up to $12 billion to restore.
"This is going to be a very serious storm, no matter what the track is, no matter how much it weakens. This is a life threatening storm to people here," Bloomberg said.
Irene's approach stirred painful memories of Hurricane Katrina, which smashed into the Gulf Coast in 2005, stranding thousands of people in New Orleans and overwhelming poorly prepared local and federal authorities.
Hurricanes are rare in the northeastern United States -- the last major hurricane to hit New York was Gloria in 1985 -- but this time authorities say they are ready.
The US military said up to 101,000 National Guard soldiers were available if needed and designated military bases in three states as staging areas.
Despite dire warnings, there were still scattered holdouts who ignored the evacuation orders and accused officials of hyping the danger.
In the neighborhood near the fun fair of Coney Island, some said they enjoyed the strangeness of seeing the summer hot spot deserted -- even if it faced possibly dangerous flooding within hours.
Don John, who lives near Coney Island, strolled down with his sister Ann to take pictures.
"I wanted to see Coney Island like it is in December, except not cold," he said.
After New York the storm was on track to batter more of New England, to the north and east, including Boston.
In Tiverton, Rhode Island, police were also working to evacuate residents from at-risk coastal zones. Again, not everyone believed there was any point.
"My neighbors are all about 74 to 78 years old and one has lost two houses to storms in her lifetime. She says she's weathering the storm and is frying fish for dinner for us," said Kathy Whalen, 44.