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.L said on Monday the Category 1 storm had strengthened as it turned toward the coast and was moving at 20 miles per hour (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 U.S. states have declared states of emergency, and with the U.S. election eight days away President Barack Obama canceled a campaign event in Florida on Monday in order to return to Washington and monitor the U.S. government's response to the storm.

"This is a serious and big storm," Obama said on Sunday 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 last week before pounding U.S. coastal areas with rain and triggering snow falls at higher elevations as it moved north.

Forecasting services indicated early Monday the center of the storm would strike the New Jersey shore near Atlantic City on Monday night. While Sandy does not 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 were at a maximum of 85 mph (140 kph), the NHC said in its 8 a.m. (1200 GMT) report, up from 75 mph (120 kph) six hours earlier. It said tropical storm-force winds reached as far as 485 miles (780 km) from the center.

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 (260 km) from the center of storm, the U.S. 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 (3.4 meters).

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.

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 (30 cm) of rain in some areas, as well as up to 3 feet (1 metre) of snowfall in the Appalachian Mountains from West Virginia to Kentucky.

At 8 a.m. (1200 GMT), the NHC said Sandy was centered about 265 miles (425 km) southeast of Atlantic City and about 310 miles south-southeast (500 km) of New York City.

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 (PSX.N) 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 (140 kph) 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." (Additional reporting by John McCrank, Edith Honan, Caroline Humer, Paul Thomasch and Janet McGurty in New York; Barbara Goldberg in New Jersey; Gene Cherry in North Carolina; Dave Warner in Philadelphia; Tom Hals in Milford, Delaware; Mary Ellen Clark and Ebong Udoma in Connecticut; Matt Spetalnick in Washington; Editing by Andrew Osborn and Eric Beech)

• A Permanent Economic Emergency