Millions without power, 15 people killed and widespread flooding reported in huge population centres in Sandy's path.
Millions of people in the eastern United States have been affected by the giant storm Sandy that has caused flooded homes, fallen trees, widespread power outages and swamped New York City's subway system.
At least 15 people were reported killed in the US by Sandy, one of the biggest storms to ever hit the country, which dropped just below hurricane status before making landfall on Monday night in New Jersey. The storm had already killed more than 60 people in the Caribbean.
More than 1 million people in a dozen states were under orders to evacuate as the massive system plowed westward.
One disaster forecasting company predicted economic losses could ultimately reach billion, only half insured.
US President Barack Obama on Tuesday declared that Sandy had triggered "major disaster" in the states of New York and New Jersey and freed up federal aid for those who lost homes or businesses.
Heavy snows threatened mountainous regions inland, and huge population centers of Baltimore, Philadelphia and Washington DC lay in the storm's path.
Michael Bloomberg, mayor of New York City, said in a press conference on Tuesday morning that the upcoming challenges facing the city in the coming days "are enormous."
"The most recent report states three quarters of a million New Yorkers are without power," he said. "You should expect, given the extent of damage, power will be out for two or three days, maybe even longer than that."
Bloomberg went on to say public transport will remain closed until further notice, and schools and airports will also remain closed on Tuesday.
"While the worst of the storm has passed, conditions are still dangerous, and I cannot stress this enough," he said.
Streets of New York City filled with floodwater, raising fears that the city's subway tunnels could be inundated, and flying debris blew along deserted sidewalks.
The city closed down subway, bus and commuter train systems as of Sunday night.
Trees were downed across the region, falling debris closed a major bridge in Boston and floodwater and gusts of wind buffeted coastal towns such as Fairfield, Connecticut, home to many commuters into New York City, where police cruisers blocked access to the beaches.
Power and back-up generators failed at New York University hospital, and patients were being elsewhere for care.
The storm's wind field stretched from the Canadian border to South Carolina, and from West Virginia to an Atlantic Ocean point about halfway between the US and Bermuda, easily one of the largest ever seen.
The National Hurricane Center said Sandy came ashore as a "post-tropical cyclone", meaning it still packed hurricane-force winds but lost the characteristics of a tropical storm. It had sustained winds of 129km per hour (kph), well above the threshold for hurricane intensity.
US stock markets were closed on Monday, the first time since the attacks on September 11, 2001, and will remain shut on Tuesday.
The federal government in Washington was also closed, and so too were schools up and down the East Coast.
New York's electric utility, Con Edison, said it expected "record-size outages". The company said it would preemptively shut down transformers in some areas to prevent further damage.
Early on Tuesday morning, US power company Excelon Corp declared an alert at the 615MW Oyster Creek nuclear power reactor in New Jersey due to rising ocean water from Sandy, a spokesman at the US Nuclear Regulatory Commission said.
An alert is the second lowest of the NRC's four emergency classifications.
The NRC spokesman said if the flood waters continued to rise, it could affect the reactor's service water pumps, which are used for shut-down cooling and to cool the spent-fuel pool.
A levee broke in northern New Jersey early on Tuesday morning, flooding the towns of Moonachie, Little Ferry and Carlstadt with 1.2 to 1.5 meters of water in the wake of Hurricane Sandy, officials told Reuters.
"We are in rescue mode," said Jeanne Baratta, chief of the Bergen County Executive. There were no reports of fatalities as of yet, she said.
Baratta said the three towns had been "devastated" by the flood of water caused by Sandy.
The storm interrupted the US presidential campaign with eight days to go before the November 6 election.
President Obama cancelled campaign event spending Monday at the White House overseeing the response to the disaster, while his Republican rival Mitt Romney also canceled some campaign appearances in response to the storm.
"This is going to be a big and powerful storm and all across the eastern seaboard I think everybody is taking the appropriate preparations," Obama told reporters after having a briefing on the storm in the White House Situation Room.
Thousands of flights were cancelled, both domestic international, and public transportation in the New York and Washington areas was suspended.