The declaration came after the massive storm battered the US East Coast, flooding lower Manhattan and coastal New Jersey and leaving millions of people across the densely populated eastern seaboard without power.
At least 14 people were killed in several states by falling trees and other storm hazards, with the toll likely to rise as rescue teams survey the damage.
The disaster declaration makes federal funding available for individuals, including grants and loans for temporary housing and home repairs.
Obama spent Monday at the White House overseeing the response to the disaster, taking a day off from the campaign trail a week before the November 6 presidential election.
His Republican rival Mitt Romney also canceled some campaign appearances in response to the storm.
750,000 customers without electricity: Bloomberg
Meanwhile New York Mayor Michael Bloomberg has said that the two biggest challenges faced by the city were restoring the mass transit system and power supply.
In a statement to the press, the mayor said that at least 750,000 customers in the city were without electricity and over 6,100 people were in shelters. He added that at least 10 people had been killed.
Bloomberg announced that schools would remain closed on Wednesday.
The massive storm stretched over hundreds of miles and paralyzed several major cities as it brought coastal flooding and hurricane-force winds to the densely-populated East Coast and blizzards to the mountainous interior.
Seawater coursed between the iconic skyscrapers of New York's financial district in lower Manhattan, flooding subways and road tunnels and shorting out the power grid, plunging more than six million households into darkness.
Further south, the sea surged over vast swathes of the eastern seaboard, turning coastal cities into ghost towns as the high winds grounded airplanes and shut down rail links, public transport and government offices.
The catastrophe completely overshadowed the US election race, forcing a halt to campaigning a week before Americans are due to go to the polls to choose between President Barack Obama and challenger Mitt Romney.
Hurricane Sandy had killed 67 people as it tore through the Caribbean, and reports of more deaths began to arrive after it made landfall at 8:00 pm (0000 GMT) in New Jersey and began to wreak havoc in the United States.
Local officials in New York, New Jersey, Pennsylvania, Maryland, West Virginia and North Carolina reported 13 dead in storm related incidents, and Toronto police said a Canadian woman was killed by flying debris.
Authorities warned the threat to life and property was "unprecedented" and ordered hundreds of thousands of residents in areas from New England to North Carolina to evacuate their homes and seek shelter.
The National Hurricane Center said wind speeds inside Sandy dropped as the storm became a post-tropical cyclone, but remained hurricane-force at 75 miles per hour (120 kmph) after it made landfall near casino resort Atlantic City.
Falling trees tore down power cables, plunging what weather experts said were millions of homes into darkness, while storm warnings cut rail links and marooned tens of thousands of travelers at airports across the region.
A nuclear power plant in New Jersey declared an alert as waters rose.
The Oyster Creek nuclear power plant, just north of Atlantic City, was already on a scheduled outage as Sandy made landfall, and the industry regulator said there was no immediate danger.
The hurricane sent a record storm surge of 13.7 feet (4.15 meters) into lower Manhattan, flooding seven major subway tunnels used by hundreds of thousands of daily commuters and swamping cars in the financial district.
"The New York City subway system is 108 years old, but it has never faced a disaster as devastating as what we experienced last night," city transport director Joseph Lhota said early Tuesday.
Hours earlier, a power sub-station exploded in a burst of light captured by amateur photographers as a massive blackout left much of Manhattan, and some 500,000 homes across New York City, in darkness.
The flood waters had begun to recede early Tuesday, but the Con Edison power company said it could take a week to completely restore power.
Disaster estimating firm Eqecat forecast that Sandy would affect more than 60 million Americans, a fifth of the population, and cause up to $20 billion (15 billion euros) in damage.
Refineries closed and major arteries such New York's Holland Tunnel were shut to traffic. The operator of two major New Jersey nuclear plants said they might have to be closed, threatening half the state's power supply.
The New York Stock Exchange, the Nasdaq and the futures markets in Chicago were closed for Monday and Tuesday, along with federal government offices and the entire Amtrak rail network on the eastern seaboard.
Obama urged Americans to heed local evacuation orders as he stepped off the campaign trail and spent the day in the White House helping to coordinate the response to the disaster.
"The election will take care of itself next week," Obama said. "Right now, our number one priority is to make sure that we are saving lives... and that we respond as quickly as possible to get the economy back on track."
Both the Democratic incumbent and his Republican rival Romney were keen to display resolute leadership in the face of the storm, given the memory of Hurricane Katrina in 2005. Romney also canceled some appearances.
Former president George W Bush was widely seen as having bungled the handling of Katrina, which devastated New Orleans. The failure of authorities in the ensuing emergency response tainted the rest of his presidency.
Obama has signed emergency declarations to free up federal disaster funds for New York state, Massachusetts, Connecticut, Maryland, the District of Columbia, New Jersey, Pennsylvania, Rhode Island, Virginia and West Virginia.
COMMENTS (15)
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@Dr Priyanka: Sorry for late reply....But there is power outage in many parts of NJ and the estimated time of power resumption could be around this weekend.....
Also since East Brunswick is closer to the Raritan Bay...They may have been evacuated mandatorily.....
I think its just a matter of time before you contact your brother :)
Areas around Toronto were hit by winds and heavy rain too. We were more affected by the cold storm though
@1984: Cant reach my brother in East Brunswick NJ by phone/email. What is the situation there?
I am in Central Jersey...Just close to the eye of the storm...The power went off and has just now came.....
I would like to thank US govt for the fact that they made necessary precautions like mandatory evacuation to make sure that the human losses are minimized......
@Something Clever. Glad your safe -- 7 million without power, 100 mph winds recorded in Jersey City, major flooding in multiple states, 16 dead, 50 houses burned to the ground, nuclear power plants taken off line, NYS stock exchange closed - etc etc. Major storm by anyone's definition.
Here's wishing Allah keeps all lives safe and comfortable.
I am 90 miles from the eye of the storm. The winds howled and twisted the trees with branches flying like projectiles. It was real scary.
The system sprung into action on a timely basis from federal to local levels, minimizing and basis mitigating the loss as much as humanly possible.
We thank all the well wishers for your prayers. May such calamities help humanity come together for the common welfare of mankind and to seek a mindful way of life in cohesion with mother nature.
Prayers and thoughts with the people affected. Infrastructure can be replaced, human lives are irreplaceable. Fortunately they are well equipped and sincere to their people, so even the poorest has some semblance of hope of something to fall back upon.
@Something Clever: that's because u live in US. If the same storm was to hit india/pakistan thousands will die.
on second thoughts that will also be only interesting. because no one will show any concern.
Pak Army has lot of experience in this area. I hope that US will invite them for assistance
@Something Clever: Please stay safe and be careful, rather than going foolhardy.
I live in northeast Pennsylvania, or to be more exact for people less familiar with the areas, basically right next to both New York and New Jersey. Radar shows me well within its range for a good size of its duration. The hurricane has been pretty boring so far. No power outages, I've seen far worse rain when hurricane conditions weren't involved but, the wind has been interesting. Nothing has been blown away. I walked outside when I've heard it sound bad but the most it was worth was a grin of acknowledgement. I'm not next to the ocean though and I'm well aware of the difference that makes. But, seeing the news and the reactions for even my area, you'd think it was doomsday. In reality at this point I can really only think of calling it "interesting" without a hint of concern attached.