Much of the eastern United States was in lockdown mode Monday awaiting the arrival of a hurricane dubbed "Frankenstorm" that threatened to wreak havoc on the area with storm surges, driving rain and devastating winds.

New York authorities ordered the evacuation Sunday of 375,000 people from low-lying coastal areas as the imminent arrival of Hurricane Sandy, which strengthened overnight and forced the entire eastern seaboard to out its normal life on hold.

More than 7,400 flights out of east coast hubs were canceled and ground transport was due to grind to a halt on as non-essential government staff were told not to show up for work.

Amtrak suspended all bus and train services up and down the coast. Subway services, buses and commuter trains were also shut down in New York, Philadelphia and Washington.

And the New York Stock Exchange, Nasdaq and the futures markets in Chicago said they will be closed on Monday, and possibly even on Tuesday.

The United Nations canceled its meetings at its New York headquarters, Broadway theaters and Carnegie Hall shut their doors, and classes were canceled at schools in Baltimore, Boston and Washington as well as a host of smaller towns.

Hundreds of thousands of residents in low-lying coastal areas were under orders to clear out and an AFP reporter said the beach resort of Rehoboth in Delaware was a ghost town as the deadline passed for mandatory evacuation.

Mindful of Hurricane Katrina, which flooded ill-prepared New Orleans in 2005, killed more than 1,800 people and left an indelible mark on George W. Bush's presidency, President Barack Obama took no chances, ordering emergency agencies to strand ready and asking people to take every possible precaution.

"My first message is to all people across the eastern seaboard, mid-Atlantic going north. You need to take this very seriously," Obama said, urging 50 million Americans across the region to heed the advice of local authorities.

The president, who spoke after being briefed at the headquarters of the Federal Emergency Management Agency (FEMA), cautioned that Sandy was a slow-moving storm that certain areas would take a long time to recover from.

"The time for preparing and talking is about over," FEMA administrator Craig Fugate warned. "People need to be acting now."

Obama signed emergency declarations to free up federal disaster funds for New York state, Massachusetts, Connecticut, Maryland, the District of Columbia, New Jersey, Pennsylvania and Rhode Island.

As some defiant New Yorkers stocked up on beer and laughed off evacuation orders saying they intended to ride out the storm, the National Weather Service office in neighboring New Jersey held no punches in its warning to residents.

"If you are reluctant to evacuate, and you know someone who rode out the '62 storm on the Barrier Islands, ask them if they could do it again," a bulletin said, referring to the notorious Ash Wednesday storm of 1962.

"If you are reluctant, think about your loved ones, think about the emergency responders who will be unable to reach you when you make the panicked phone call to be rescued, think about the rescue/recovery teams who will rescue you if you are injured or recover your remains if you do not survive."

Fearful residents from Washington to New York to Boston queued for emergency provisions like bottled water and batteries in long lines that stretched out the doors of supermarkets.

-- A 'nor-easter on steroids' --

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After laying waste to parts of the Caribbean, where it claimed 66 lives, most of them in Cuba and Haiti, Hurricane Sandy was predicted to come crashing ashore in New Jersey and Delaware late Monday or early Tuesday.

The storm strengthened overnight, reaching hurricane force winds upwards of 85 miles per hour (140 kilometers) per hour, up from 75 miles (120 kilometers) an hour late Sunday, the National Hurricane Center said.

It was now about 385 miles (615 kilometers) south southeast of New York early Monday and beginning to turn west.

Winds stretched more than 485 miles (780 kilometers) from its eye, the center pointed out, which means everywhere from South Carolina to southern Canada was due to be affected.

"The system is so large that I would say millions of people are at least in areas that have some chance of experiencing either flash flooding or river flooding," National Hurricane Center director Rick Knabb warned.

Forecasters cautioned that the massive storm was far larger and more dangerous than last year's devastating Hurricane Irene that claimed 47 lives and caused an estimated $15 billion in damage.

Current projections show Sandy barreling north on a collision course with two other weather systems that would send it hooking into the Delaware or New Jersey coast as one of the worst storms on record.

Weather experts say the collision could create a super-charged storm bringing floods, high winds and even heavy snow across a swath of eastern states and as far inland as Ohio.

"Sandy will be more like a large nor'easter on steroids," warned Alex Sosnowski, a senior meteorologist for Accuweather.com, as experts predicted widespread damage, mass power outages for days and disastrous flooding.

Forecasters warned that New York Harbor and the Long Island Sound could see seawater surges of up to 11 feet (more than three meters) above normal levels.

Television images from North Carolina's Outer Banks, a chain of low lying islands, showed wild surf and torrential rain hitting the coast, while residents in Virginia were already reporting coastal flooding.

Governors have declared states of emergency in Connecticut, Delaware, Maryland, Massachusetts, New Jersey, New York, North Carolina, Pennsylvania, Vermont, Virginia and the US capital Washington.