(Reuters) - Hurricane Sandy, a mammoth storm menacing the East Coast, took aim at the most densely populated U.S. region on Monday, forcing hundreds of thousands to seek higher ground, halting public transport and closing schools, businesses and government departments. About 50 million people from the Mid-Atlantic to Canada were in the path of the storm, which forecasters say could be the largest ever to hit the U.S. mainland. It is expected to topple trees, damage buildings and cause widespread power outages over the next few days. Sandy, which killed 66 people in the Caribbean and has brought lashing rains to coastal areas and snow at higher elevations, will cause extensive flooding when it moves inland, forecasters said. The websites of forecasting services indicated early Monday the storm will strike the New Jersey shore near Atlantic City Monday night. While Sandy does not pack the punch of Hurricane Katrina, which devastated New Orleans in 2005, its winds stretch some 520 miles from its eye, meteorologists said. 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 Barack Obama and Republican challenger Mitt Romney to cancel some campaign stops and fueled fears that 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 are 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 2 a.m. (0600 GMT), the National Hurricane Center said Sandy was centered about 425 miles south southeast of New York City. The storm was turning toward the coast and moving at 14 mph and would bring a "life-threatening" surge, hurricane-force winds and heavy snows in the Appalachian mountains. 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 is grinding to a halt, with airlines cancelling flights, bridges and tunnels closing, and Amtrak scrapping nearly all of its passenger r