Howling winds, furious tides and a series of torrential downpours announced the arrival of Cyclone Sandy on the American east coast last night, an unwelcome guest ahead of Halloween that has forced mass evacuations across what is the country’s most densely populated region.

The storm, a sprawling weather-monster that has led to declarations of emergency in more than half a dozen northeastern states, crashed ashore near Atlantic City in southern New Jersey at around 8pm, according to the National Hurricane Center. With wind speeds easing slightly to 80 mph as it made the transition to land, the weather system was also reclassified as a Post-Tropical Cyclone.

Earlier on Monday, what was then Hurricane Sandy was producing maximum sustained winds of 90 mph, up from 75 mph on Sunday night, as it swept across the Atlantic.

As it approached land last night, police in midtown Manhattan rushed to cordon off streets around a partially-completed skyscraper after sharp winds damaged a crane and left it dangling perilously from a height of several dozen stories above ground.

The incident at 157 West 57th Street, the site of what is poised to be the tallest residential tower on the island, triggered fresh evacuations in the nearby area, with fears that the crane could come crashing down as the weather deteriorated. The facade of another building in Manhattan’s Chelsea neighborhood was also reported to have been damaged last night.

Sandy had begun to makes it presence felt hours earlier, sparking flooding in Atlantic City and in Ocean City, Maryland, with swelling waters washing away parts of the latter’s famous pier.

Coastguards, meanwhile, rescued 14 of the 16 crew of the tall ship HMS Bounty after they were forced to abandon the replica vessel in heavy seas about 90 miles of the North Carolina coast.

Tens of millions of people across Virginia, Maine, Massachusetts, New York, Delaware, Pennsylvania, Rhode Island and other north-eastern states were told to hunker down as forecasters also predicted heavy snow across the region. They said a major risk was from the record-breaking span of the storm, with hurricane-force winds extending up to 175 miles from its centre in the afternoon, while tropical storm-force winds extended up to 485 miles.

President Barack Obama, breaking away from the election campaign and assuming the role of commander-in-chief, warned that “millions of people are going to be affected”.

“The most important message I have for the public right now is please listen to what your state and local officials are saying,” he added. “When they tell you to evacuate, evacuate.”

Bracing for the impact, officials had by yesterday morning ordered the evacuation of 375,000 people in New York City, 50,000 in Delaware and 30,000 in Atlantic City. Nearly 9,000 flights were cancelled and power cuts were reported in New Jersey, Virginia, Delaware, Maryland, Connecticut, Rhode Island and New York state. At least nine states, along with the District of Columbia, declared emergencies.

Martin O’Malley, the Governor of Maryland, sounded a grim warning yesterday, saying: “The days ahead are going to be difficult. There will be people who die and are killed in this storm.”

Millions of pupils were given a day off as schools closed. In Washington DC, which was lashed by heavy rain and sharp winds, the entire metro system was shut down.

Earlier, New York City’s mayor, Michael Bloomberg, announced the shutdown of the country’s biggest urban transportation system. All 468 of the city’s subway stations began closing at 7pm on Sunday – for the second time in just over a year, after they were temporarily closed ahead of Hurricane Irene.

He also ordered a mandatory evacuation of low-lying areas around the city. He said 16,000 beds had been set up in emergency shelters at 76 schools across the city. However, only 3,000 people spent Sunday night in shelters.

“Conditions are deteriorating very rapidly and the window for getting out safely is closing,” Mr Bloomberg added. “It’s getting too late to leave.” New Yorkers appeared to heed his warning, and the streets of lower Manhattan were deserted yesterday, Sandbags lined the approaches to the New York Stock Exchange in Wall Street, which will remain out of out of action today.

Further north, in Manhattan’s usually bustling West Village neighbourhood, shops and restaurants had their shutters down. Barriers had been put up at the entrance to Washington Square, and Sullivan Street was mostly empty, save for the odd passing dog-walker or jogger.