Millions of people are without power in the U.S. northeast as the powerful post-tropical storm Sandy pummels the region with hurricane-force winds, rain, and a dangerous storm surge that has caused flooding in many coastal areas — including parts of New York City.

Sandy has caused mass disruption in coastal communities and beyond, as thousands of flights were cancelled, trains disrupted, roads closed and schools and offices shut down as the storm approached.

The storm was tracking inland around 11 p.m. ET, the U.S. National Hurricane Center said, but maximum sustained winds were still strong, clocking in around 120 km/h.

"On the forecast track, the centre of Sandy is expected to move across Pennsylvania during the next day or so ... then move into western New York Tuesday night," the Miami-based centre said in a statement.

At least 10 deaths in five states have been blamed on the storm, The Associated Press reported. A woman was also killed in Canada, when she was struck by flying debris in Toronto amid gusting winds.

"Do not go outside. Conditions are extremely dangerous," New York City Mayor Michael Bloomberg said on Twitter as the storm lashed the city Monday night. "Please stay where you are until the storm passes."

He urged people to stay off the roads and leave them clear for emergency workers, who have been dealing with an onslaught of 911 calls.

"There are about 60 blocks in Manhattan now totally without power," CBC's David Common said from New York City late Monday night. "We understand that exists not only across New York City, but right up and down the northeast coast — huge numbers of people without power."

New York University hospital was struck by a power outage after losing backup power, the mayor said, noting that the city is working with the hospital to try to move people out.

Media reports suggest as many as three million customers in the U.S. may be without electricity as Sandy downs lines and disrupts the distribution of power.

Storm surge

Ahead of the storm, people were urged to brace for a powerful storm surge, and officials cautioned people to stay away from beaches, boardwalks and coastal roads as Sandy neared.

A record four-metre surge of seawater pushed floodwater into Lower Manhattan and parts of the financial district, AP reported.

"The West Side Highway is no longer a highway — it's a river with a current," Common said, and water was also seen running along roads in Queens, Brooklyn and beyond.

Water began pooling in rail yards and on highways near the Hudson River waterfront on Manhattan's far west side. On coastal Long Island, floodwaters swamped cars, downed trees and put neighbourhoods under water as beachfronts and fishing villages bore the brunt of the storm.

Before the storm moved in, thousands of people in low-lying areas had been urged to leave. Those who stayed in New York had few ways to get out. New York's subways, which serve five million people a day, were shut down. The Holland Tunnel connecting New York to New Jersey was closed, as was a tunnel between Brooklyn and Manhattan, and the city shut down the Brooklyn Bridge, the George Washington Bridge, the Verrazano-Narrows Bridge and several other spans because of high winds.

New Jersey landfall

Sandy made landfall near Atlantic City, N.J., around 8 p.m. ET, the U.S. National Hurricane Center said. By 9 p.m., the centre said hurricane-force wind gusts had been reported over Long Island and the New York City area.

Just before its centre reached land, the storm was stripped of hurricane status and reclassified as a post-tropical cyclone, but the distinction was purely technical, based on its shape and internal temperature. It still packed hurricane-force wind, and CBC meteorologist Johanna Wagstaffe cautioned that "regardless of exact landfall or classification, this is still a massive storm with massive impacts."

High winds blow sea foam onto Jeanette's Pier in Nags Head, North Carolina, one of several states that declared states of emergency. (Gerry Broome/AP)

The storm has washed away a section of the Atlantic City boardwalk in New Jersey, where casinos were closed as Sandy neared.

"This storm is causing a great deal of damage. Especially on the New Jersey coastline," New Jersey Gov. Chris Christie said on Twitter earlier in the evening.

Christie said he spoke with U.S. President Barack Obama earlier in the day to discuss the state's needs as the storm approached.

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In a news conference, Christie expressed concern that some people in coastal areas had not followed orders to evacuate before the storm made landfall.

"I hope and pray there will not be a loss of life because of people's decisions to stay," he said on Twitter.

In Delaware, Gov. Jack Markell said an emergency response plan was in place to help people affected by the storm.

Storm structure changes, risk remains

Sandy's reclassification as a post-tropical cyclone was part of a transition into a more diffuse storm that is bigger and sloppier, even as the wind speed weakened.

"It's a change in structure, so it's no longer gaining its energy from the warm waters — it's now gaining its energy from temperature differences," Wagstaffe said after the storm was reclassified Monday night.

Wagstaffe said the mechanics of the storm are now the same as a nor'easter or a low-pressure system that we might see in Canada — but much larger.

She noted the transition in the structure of the storm was expected, adding that the change does nothing to change the power of the storm and may even spread its reach farther.

Earlier the storm caused flooding from North Carolina up the coast to eastern Long Island, N.Y. In Virginia Beach and Norfolk, Va., streets were inundated, marooning cars in more than half a metre of water. The surging Atlantic Ocean overwhelmed a highway on Hatteras Island in North Carolina and washed over the main street in Sea Bright, N.J.

New York, Philadelphia, Washington and Baltimore shut down their subways, buses and trains, and schools were closed on Monday. Boston also called off school. And all non-essential government offices were closed in Washington, D.C.

'Citizens have a duty in this too'

New York Gov. Andrew Cuomo said a major concern is floodwaters entering the New York City subway system, where the salt water from the Atlantic Ocean could cause corrosion to train brakes, track control systems and power lines.

"Citizens have a duty in this too," Cuomo said. "They need to be smart, they need to use common sense. Citizens do not need to be on the road — they need to leave the roads free" for emergency vehicles, he added.

The New York Stock Exchange was shut down for Monday, including electronic trading, and will remain closed Tuesday. Nasdaq shut the Nasdaq Stock Market and other U.S. exchanges and markets it owns, although its exchanges outside the U.S. operated as scheduled.

Several companies have postponed reporting their earnings as a result, including Pfizer Inc. and Thomson Reuters.

"I think this one's going to do us in," said Mark Palazzolo, who boarded up his bait-and-tackle shop in Point Pleasant Beach, N.J., with the same wood he used in past storms, crossing out the names of hurricanes Isaac and Irene and spray-painting "Sandy" next to them.

"I got a call from a friend of mine from Florida last night who said, 'Mark, get out! If it's not the storm, it'll be the aftermath. People are going to be fighting in the streets over gasoline and food,'" Palazzolo said before the storm hit.

Sandy was blamed for at least 65 deaths in the Caribbean before it began travelling northward, parallel to the eastern seaboard.

As the storm approached, Obama and Republican presidential hopeful Mitt Romney suspended their campaigning with just over a week to go before election day.

In Washington, Obama declared emergencies in Massachusetts, Connecticut, Rhode Island, New York, New Jersey and Pennsylvania, authorizing federal relief work to begin well ahead of time. He promised the government would "respond big and respond fast" after the storm hits.

"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. We are not going to get bogged down with a lot of rules," he said.

The United Nations closed its headquarters in Manhattan on Monday and cancelled all meetings there.

With files from The Associated Press