A deluge of water floods the Battery Tunnel in Manhattan as superstorm Sandy made its approach in New York, Oct 29, 2012. [Photo/CFP]

NEW YORK/REHOBOTH BEACH, Del. - Sandy, one of the biggest storms to hit the United States, pounded the east coast on Tuesday, flooding large parts of New York City, bringing transport to a halt and interrupting the presidential campaign.

More than 5.5 million people were left without electrical power by the storm, which crashed ashore late on Monday near the gambling resort of Atlantic City, New Jersey. More than one million people across a dozen states were ordered to evacuate.

Heavy snows threatened mountainous regions inland, and huge population centers of Baltimore, Philadelphia and Washington lay in the storm's path.

A gas station is submerged in floodwaters near the Gowanus Canal in the Brooklyn Borough of New York, Oct 29, 2012. [Photo/Agencies]

"We have not seen the kind of flooding problems that certainly could have happened thus far, but we've still got a long ways to go to get through this storm," Washington Mayor Vincent Gray said on local television.

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.

In lower Manhattan, firefighters used inflatable orange boats to rescue utility workers trapped for three hours by rising floodwaters inside a power substation.

One of the Con Ed workers pulled from the floodwater, Angelo Amato, said he was part of a crew who had offered to work through the storm.

"This is what happens when you volunteer," he said.

Two people were killed in the New York borough of Queens - a man in a house hit by a falling tree and a woman who stepped into an electrified puddle of water. Massachusetts police said one man was killed in Peabody in an accident related to the bad weather. Toronto police also recorded one death, a woman hit by flying debris.

Power and back-up generators failed at New York University hospital, and patients were being elsewhere for care.

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.

"People are definitely not taking this seriously enough," said police officer Tiffany Barrett. "Our worst fear is something like Katrina and we can't get to people."

A blacked out New York City skyline is seen in New York, Oct 29, 2012, as Sandy made landfall in the northeastern United States. [Photo/Agencies]