Sandy slammed into the New Jersey coastline as a post-tropical cyclone, killing at least 16 people, flooding cities from Virginia to New York City and leaving millions without power early Tuesday morning.

Officials said the storm system is to blame for at least 16 deaths in seven states. Some of the victims were killed by trees knocked over by powerful winds.

The storm made landfall in New Jersey around 8 p.m. ET Monday, and immediately hammered the region with 130 km/h winds and heavy rains, leading to widespread flooding and power outages.

As many as 6.2 million people were without power early Tuesday.

Although the storm was downgraded from a hurricane earlier Monday evening, Sandy was still a powerful and dangerous weather system, threatening a 1,300-kilometre region and up to 50 million people in several states.

The weather system flooded streets from Virginia to Atlantic City, where it washed away part of the famous boardwalk.

A nearly four-metre storm surge also washed into Lower Manhattan, flooding streets.

The city had to move staff and about 200 patients out of New York University’s Tisch Hospital, which lost its backup power. Patients on respirators, as well as 20 babies from the neonatal intensive care unit, were moved to other facilities.

Forecasters warned of not only heavy rains and high winds, but also as much as one metre of snow in some states, including West Virginia.

By 3:30 a.m. Tuesday:

The storm had maximum sustained winds of 120 km/h

6.2 million people were without power across the U.S. northeast

More than 12,000 flights were cancelled

The highest storm surge measured 3.8 metres, at Kings Point, N.Y.

After Con Ed cut power to Lower Manhattan, other parts of New York went dark as floodwaters damaged power infrastructure.

"We knew that this was going to be a very dangerous storm, and the storm has met our expectations," New York Mayor Michael Bloomberg said. "This is a once-in-a-long-time storm."

Earlier Monday, U.S. President Barack Obama declared states of emergency in Massachusetts, Connecticut, Rhode Island, New York, New Jersey and Pennsylvania, and released federal relief to those areas ahead of the storm.

"Please listen to what your state and local officials are saying,” Obama said from the White House earlier Monday. “When they tell you to evacuate, you need to evacuate. Don't delay, don't pause, don't question the instructions that are being given, because this is a powerful storm."

While it could take days to determine the extent of the damage, estimates put the potential price tag anywhere between $10 billion and $20 billion, which could make it one of the costliest in U.S. history.

The storm halted train and air travel through much of the northeast. More than 12,000 flights were cancelled for Monday and Tuesday across the northeast, which had a ripple effect across the U.S. and for Trans-Atlantic flights. By Monday night, all New York City airports were closed.

Amtrak began cancelling service over the weekend and had cancelled nearly all trains over the eastern seaboard by Monday.

Schools, businesses, transit systems and some highways were closed from Washington, D.C. to Boston.

New York Under Water

Over the weekend, tens of thousands of people were ordered to leave coastal areas of New York, New Jersey and Delaware, while all 12 of Atlantic City’s casinos were closed

The New York Stock Exchange also suspended operations Monday and later said the closure would continue into Tuesday. It’s the first unplanned shutdown of the stock exchange since the 9-11 attacks.

The United Nations also shut down Monday.

Bloomberg pleaded with residents not to go outside, drive or put themselves in danger otherwise. He said the city’s 911 emergency service was receiving up to 10,000 calls each half hour, mostly about downed trees and power lines.

“Please, don’t call 911 unless it’s a life-threatening situation,” he said Monday night, urging residents to report storm-related damage to the city’s 311 service.

Bloomberg also warned thrill-seekers from trying to surf in the dangerously high waves or coming close to shore to document the storm’s landfall.

If a first-responder loses his or her life trying to save someone who was being “irresponsible,” that would be an “outrage,” Bloomberg said.

In and around the city:

Water flooded into Manhattan from three sides, flooding streets, knocking down trees and carrying away cars.

Bridges and tunnels into and out of Manhattan were closed.

Earlier, a construction crane on top of a mid-town highrise collapsed, but no one was injured.

The façade of a four-story building in Chelsea crumbled and fell away. No one was injured, but a nearby car was damaged.

On the U.S. presidential election front, both Obama and Republican presidential candidate Mitt Romney suspended campaigning Monday, with just a week left before voting day.

Early voting was also cancelled in Maryland and Washington, D.C.