Forecasters in Miami say the center of what was Hurricane Sandy has made landfall along the New Jersey coast near Atlantic City.
The National Hurricane Center says the storm packing torrential rains and wind roared ashore about 8 p.m. local time. The vast storm has already knocked out electricity to more than 1.5 million people and figured to upend life for tens of millions more.
Heavy flooding has begun in New York and along America's East Coast. The 1,000-mile-wide monster storm, which forecasters said could be the largest to ever hit the US mainland, has only strengthened as it bounds towards a trail of major cities across nine states all already facing a state of emergency.
The Category-1 storm is expected to generate a "life-threatening storm surge", having already killed more than 60 people as it pummelled the Caribbean last week.
Carrying winds of up to 85mph, Sandy is expected to topple trees, damage buildings, cause power outages and trigger even heavier flooding, generating storm water walls of up to 11-feet.
The center of the storm was expected to strike the New Jersey shore near Atlantic City tonight, with the city's famous boardwalk already battered by an onslaught of water. Some 30,000 had left their homes in the city.
A further 50,000 were forced to flee in the state of Delaware, while tens of thousands of people in coastal towns and low-lying areas from Maryland to Connecticut evacuated as the hurricane nears land.
Sandy is expected to collide with two other weather systems: a wintry storm moving in from the West and cold air streaming down from the Arctic, generating a superstorm already dubbed "Frankenstorm".
Floodwater has already lapped over barriers in New York's lower Manhattan, now a ghost town following a major evacuation drive, before the state's governor called in an extra 1,000 national guards to deal with the anticipated storm surge.
New York and Philadelphia earlier both shut down their subways, buses and commuter trains and announced that schools were closed for the day at least. School was also out in Boston, Washington and Baltimore.
In Manhattan, Broadway theatres and the UN building were shut, while the US stock market was closed for the first time in 27 years.
Airlines cancelled more than 7,600 flights, with US-bound planes grounded around the world, and Amtrak suspended passenger trains across the north east.
With the US election only eight days away, President Barack Obama pulled out of a campaign event in Florida and another a day later in Wisconsin in order to return to Washington and monitor the US government's response to the storm.