Hurricane Sandy, the monster storm bearing down on the East Coast, strengthened on Monday after hundreds of thousands moved to higher ground, public transport shut down and the stock market suffered its first weather-related closure in 27 years.

About 50 million people from the Mid-Atlantic to Canada were in the path of the nearly 1,600-km-wide storm, which forecasters said could be the largest to hit the mainland in US history. It was expected to topple trees, damage buildings, cause power outages and trigger heavy flooding.

The US National Hurricane Center (NHC) said on Monday the Category 1 storm had strengthened as it turned toward the coast and was moving at 32 km per hour. It was expected to bring a "life-threatening storm surge," coastal hurricane winds and heavy snow in the Appalachian Mountains, the NHC said.

Nine US states have declared states of emergency, and with the US election eight days away President Barack Obama canceled a campaign event in Florida on Monday in order to return to Washington and monitor the US government's response to the storm.

State governors warned of the acute danger from the winds and torrential rains. "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.

The U.S. stock market suffered its first weather-related closure in 27 years and many schools and businesses were closed in Washington, Baltimore, Philadelphia and New York.

While the center of the storm was not expected to make landfall until Monday night near Atlantic City, New Jersey, it was already creating dangerous conditions and forcing rescue workers into action.

Seventeen people from the replica HMS Bounty abandoned ship while stranded at sea off North Carolina in the path of the hurricane, roughly 160 miles from the center of storm, the US Coast Guard said on Monday.

"The 17-person crew donned cold water survival suits and lifejackets before launching in two 25-man lifeboats with canopies," the Coast Guard said, adding it was determining which aircraft or vessel was best-placed to launch a rescue.

The three-masted tall ship was built for the 1962 movie "Mutiny on the Bounty."

New York and other cities and towns closed their transit systems and ordered mass evacuations from low-lying areas ahead of a storm surge that could reach as high as 11 feet.

All US stock markets will be closed on Monday and possibly Tuesday, the operator of the New York Stock Exchange said late on Sunday, reversing an earlier plan that would have kept electronic trading going on Monday.

The United Nations, Broadway theaters, New Jersey casinos, schools up and down the Eastern Seaboard, and myriad corporate events were also being shut down.

Transportation systems shut down in anticipation. Airlines canceled flights, bridges and tunnels closed, and national passenger rail operator Amtrak suspended nearly all service on the East Coast.

The US government told non-emergency workers in Washington, D.C., to stay home.

Utilities from the Carolinas to Maine reported late Sunday that a combined 14,000 customers were already without power.

The second-largest oil refinery on the East Coast, Phillips 66's 238,000 barrel per day (bpd) Bayway plant in Linden, New Jersey, was shutting down and three other plants cut output as the storm affected operations at two-thirds of the region's plants.

Oil prices slipped on Monday, with Brent near $109 a barrel. "With refineries cutting runs, we're likely to see a build-up in crude stocks which could be driving bearish prices at the moment," said Michael Creed, an economist at National Australia Bank in Melbourne.

New York City Mayor Michael Bloomberg ordered the evacuation of some 375,000 people from low-lying areas of the city, from upscale parts of lower Manhattan to waterfront housing projects in the outer boroughs.

While Sandy's 85 mph winds were not overwhelming for a hurricane, its exceptional size means the winds will last as long as two days.

"This is not a typical storm," Pennsylvania Governor Tom Corbett said. "It could very well be historic in nature and in scope."-Reuters