NEW YORK (Reuters) - 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 1,000-mile-wide (1,600-km-wide) storm, which forecasters said could be the largest to hit the mainland in U.S. history. It was expected to topple trees, damage buildings, cause power outages and trigger heavy flooding.

The U.S. National Hurricane Center (NHC) said on Monday the Category 1 storm had strengthened as it turned toward the coast and was moving at 15 mph. It was expected to bring a "life-threatening storm surge", coastal hurricane winds and heavy snow in the Appalachian Mountains, the NHC said.

Nine U.S. states have declared states of emergency and President Barack Obama has warned the nation to brace itself.

"This is a serious and big storm," Obama said after a briefing at the federal government's storm response center in Washington. "We don't yet know where it's going to hit, where we're going to see the biggest impacts.

Sandy killed 66 people in the Caribbean before pounding U.S. coastal areas with rain and triggering snow falls at higher elevations.

Forecasting services indicated early Monday the storm would strike the New Jersey shore near Atlantic City on Monday night. While Sandy does not yet pack the punch of Hurricane Katrina, which devastated New Orleans in 2005, it could become more potent as it approaches the U.S. coast.

Winds increased to a maximum of 85 mph, the NHC said in its 5 a.m. (1100 GMT) report, up from 75 mph three hours earlier. It said tropical Storm force winds reached as far as 485 from the center.

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

All U.S. 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.

Sandy forced President Obama and Republican challenger Mitt Romney to cancel some campaign stops and fueled concern it could disrupt early voting - encouraged by the candidates this year more than ever - before the November 6 election.

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

'DON'T BE STUPID'

Officials ordered people in coastal towns and low-lying areas to evacuate, often telling them they would put emergency workers' lives at risk if they stayed.

"Don't be stupid, get out, and go to higher, safer ground," New Jersey Governor Chris Christie told a news conference.

Forecasters said Sandy was a rare, hybrid "super storm" created by an Arctic jet stream wrapping itself around a tropical storm, possibly causing up to 12 inches of rain in some areas, as well as up to 3 feet of snowfall in the Appalachian Mountains from West Virginia to Kentucky.

At 5 a.m. (1100 GMT), the NHC said Sandy was centered about 385 miles south southeast of New York City, about 40 miles closer than was reported three hours earlier.

Worried residents in the hurricane's path packed stores, searching for generators, flashlights, batteries, food and other supplies in anticipation of power outages. Nearly 284,000 residential properties valued at $88 billion are at risk for damage, risk analysts at CoreLogic said.

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 U.S. 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.

The U.S. Coast Guard responded to a distress call from HMS Bounty, a tall ship with 17 people aboard approximately 90 miles southeast of Hatteras, North Carolina. A replica built for the 1962 movie version of "Mutiny on the Bounty," the three-mast, 180-foot (55-metre) vessel was believed to be taking on water and was without propulsion, the Coast Guard said.

"We are talking with the crew and the owner about search and rescue possibilities," Petty Officer David Weydert said.

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," said Pennsylvania Governor Tom Corbett. "It could very well be historic in nature and in scope."