* Hurricane expected ashore Monday evening in New Jersey
* Could be biggest to hit U.S. mainland
* New York, Philadelphia and Washington in its path
* New York Stock Exchange closed Monday and Tuesday
* Early warnings of $20 billion in economic losses
NEW YORK/REHOBOTH BEACH, Del., Oct 29 (Reuters) - Hurricane Sandy, one of the biggest storms ever to hit the United States, battered the densely populated East Coast on Monday, shutting down transportation, forcing evacuations in flood-prone areas and interrupting the presidential election campaign.
Fierce winds and flooding were expected along hundreds of miles of Atlantic coast and heavy snows were forecast farther inland at higher elevations when the center of the storm moves ashore Monday evening near Atlantic City, New Jersey.
U.S. stock markets were closed for the first time since the attacks of Sept. 11, 2001, and will remain shut on Tuesday. The government in Washington was closed and school was cancelled up and down the East Coast.
Nearly 700,000 customers were without power by midday and millions more could lose electricity. One disaster forecasting company predicted economic losses could ultimately reach $20 billion, only half of it insured.
``This is going to be a big and powerful storm and all across the Eastern Seaboard I think everybody is taking the appropriate preparations,'' President Barack Obama said at the White House.
State governors from Virginia to Massachusetts warned of the acute danger from the storm for the 60 million residents in its path. Ten states have declared a state of emergency.
``There will undoubtedly be some deaths that are caused by the intensity of this storm, by the floods, by the tidal surge, by the waves. The more responsibly citizens act, the fewer people will die,'' Maryland Governor Martin O'Malley told reporters.
Forecasters said Sandy could be the largest storm to hit the mainland in U.S. history.
Off North Carolina, the U.S. Coast Guard rescued 14 of the 16 crew members who abandoned the replica tall ship HMS Bounty, using helicopters to lift them from life rafts. The Coast Guard continued to search for two missing crew members.
In New York, a crane atop a building on 57th Street in Manhattan had partially collapsed, leaving it dangling high above the street. Police said they were closing the area to pedestrians.
The storm interrupted the presidential campaign with eight days to go before the election.
Obama canceled a campaign event in Florida on Monday so he could return to Washington and monitor the government response to the storm. Republican presidential candidate Mitt Romney canceled stops Monday night and Tuesday.
Sandy picked up speed as it raced northwest toward the U.S. coast at 28 miles per hour (45 km per hour) on Monday afternoon, with top sustained winds at 90 mph (150 kph), the U.S. National Hurricane Center said.
At 2 p.m. (1800 GMT) the center of the storm was about 110 miles (180 km) southeast of Atlantic City, New Jersey, or 175 miles (280 km) south-southeast of New York City. Hurricane-force winds were already being recorded on the New Jersey coast.
``The center of Sandy is expected to make landfall along or just south of the southern New Jersey coast by early evening,'' NHC forecasters said, adding they expected little change in strength before then.
Forecasters said Sandy was a rare, hybrid ``super storm'' created by an Arctic jet stream wrapping itself around a tropical storm.
The combination of those two storms would have been bad enough, but meteorologists said there was a third storm at play - a system coming down from Canada that would effectively trap the hurricane-nor'easter combo and hold it in place.
Moreover, the storm was coming ashore at high tide, which was pulled even higher by a full moon.
While Sandy does not pack the punch of Hurricane Katrina, which devastated New Orleans in 2005, it has been gathering strength. It killed 66 people in the Caribbean last week before pounding U.S. coastal areas as it moved north.
'SOMETHING LIKE KATRINA'
In Fairfield, a Connecticut coastal town and major commuter point into Manhattan, police cruisers blocked the main road leading to the beaches and yellow police tape cordoned off side entrances. Beach pavilions were boarded up with plywood, and gusts of wind rocked parked cars.
``People are definitely not taking this seriously enough,'' police officer Tiffany Barrett, 38, said. ``Our worst fear is something like Katrina and we can't get to people.''
Further south, several feet of water flooded streets in Rehoboth Beach, Delaware. Police knocked on doors, reminding people there was a mandatory evacuation. While the police took names, they allowed residents to stay at their own risk.
Besides rain, the storms could cause up to 3 feet (1 metre) of snowfall in the Appalachian Mountains from West Virginia to Kentucky. Some people in that part of the country did not have to go to work because of the storm and used the time to vote.
At the Berkeley County Courthouse in Martinsburg, West Virginia, early voting for the Nov. 6 elections was going ahead despite the bad weather, with some 600 people casting ballots by about 1:45 p.m. (1745 GMT)
``More (people) came out today than what I anticipated but a lot of people are off work,'' Bonnie Woodfall, chief deputy for voter registration, said after fielding a flurry of calls about whether the polls should stay open. ``It's neat.''
New York and other cities closed their transit systems and schools, ordering mass evacuations from low-lying areas ahead of a storm surge that could reach as high as 11 feet (3.4 meters).
By early Monday, water was already topping the seawall in Manhattan's Battery Park City, one of the areas ordered evacuated by Mayor Michael Bloomberg.
He ordered 375,000 New Yorkers to evacuate and told those who remained to leave immediately. ``Conditions are deteriorating rapidly and the window for you getting out safely is closing.''
New York electric utility Con Edison said it expected ``record-size outages,'' with nearly 35,000 customers in lower Manhattan and Brooklyn likely to be impacted. The company is facing both falling trees knocking down power lines from above and flood waters swamping underground systems from below.
All U.S. stock markets were closed on Monday and will remain shut on Tuesday, with a plan to re-open on Wednesday that depends on conditions after the storm passes.
The United Nations, Broadway theaters and New Jersey casinos were forced to close and more than two-thirds of East Coast oil refining capacity was in the process of shutting down.
Airlines canceled flights, bridges and tunnels closed, and national passenger rail operator Amtrak suspended nearly all service on the East Coast. The U.S. government told non-emergency workers in Washington, D.C., to stay home.
Up and down the coast, worried residents in the hurricane's path packed stores, searching for generators, flashlights, batteries, food and other emergency supplies.
Johnny Lopez, an owner of Best Buy Wines and Liquors in Brooklyn, said he plans - ``God help us!'' - to stay open all day on Monday and Tuesday.
``Crazy busy yesterday,'' he said. ``It was like Thanksgiving.''