Hurricane Sandy, the "super storm" menacing the East Coast of the United States, made landfall shortly after 5pm local time [0000 GMT] over the state of New Jersey.
Hundreds of thousands of people have already been evacuated from the storm's path, and millions more are bracing for a major impact. The National Hurricane Center said the storm's top sustained winds are holding at about 150 kph; the vast storm was moving at around 45 kph.
Flooding and other damage was already reported in cities and towns along the eastern seaboard, including in New York, the most densely populated city in the United States. Cars could be seen floating down Wall Street, the heart of the city's financial district; further uptown, a four-story apartment building collapsed.
A 30-year-old man was killed after being pinned by a falling tree in New York; he is the first reported fatality from the storm in the United States.
Barack Obama, the US president, warned people to listen to local and state officials on whether or not to evacuate.
"This is going to be a big and powerful storm and all across the Eastern Seaboard I think everybody is taking the appropriate preparations," he told reporters after having a briefing on the storm in the White House Situation Room.
"This is a serious storm and it could potentially have fatal consequences if people haven't acted quickly."
Scrapping campaign plans to return to Washington, Obama sought to show voters just eight days before the November 6 election that he was giving top priority to his presidential duties in a looming national crisis, rather than his bid for re-election in a tight race.
He also appeared determined to demonstrate that his administration had learned the lessons of White House predecessor George W Bush's botched handling of Hurricane Katrina in 2005, which inflicted serious political damage.
Rushing back from a campaign visit to Florida, Obama huddled with top aides in the White House Situation Room for an update on Hurricane Sandy, which started to batter the densely populated East Coast as one of the biggest storms to ever hit the US mainland.
Hurricane Sandy is continuing to head towards the area as residents make last-minute preparations.
Sandy could be the biggest storm to hit the US mainland when it comes ashore on Monday night, bringing strong winds and dangerous flooding from the mid-Atlantic states to New England, forecasters said on Sunday.
The storm could have a brutal impact on major cities in the target zone such as Boston, New York, Washington, Baltimore and Philadelphia, in one of the most densely populated regions of the country.
New York City's subway, bus and train services were suspended on Sunday evening, bringing the country's financial centre to a virtual standstill.
Michael Bloomberg, New York mayor, also ordered public schools to close on Monday and said that areas of the city from City Island to Coney Island to Battery Park City were under mandatory evacuation.
"We are ordering the evacuation for the safety of the approximately 375,000 people who live in these areas," he said.
Stock market executives, regulators and brokers have agreed to close stock markets on Tuesday due to the hurricane.
The stock market's closure on Monday was its first weather-related shutdown in 27 years.
Every effort is being made to reopen markets on Wednesday, but a final decision has not been made yet, a source said.
The decision involved senior leadership of the exchanges, executives from brokerage firms and officials from the US Securities and Exchange Commission.
"It doesn't make sense to put people in harm's way or to only have half a market," said Nicholas Colas, chief market strategist at ConvergEx Group in New York.
"If just the electronic market was open, that wouldn't provide enough interest, with everything else still closed."
The hurricane forced the evacuation of tall ship HMS Bounty, a replica of the ship famed for the 1789 mutiny, approximately 145km off the North Carolina coast
US Coast Guard officers told reporters the 17-member crew of the training ship, used in the Pirates of the Caribbean movies, managed to get on board two life-rafts. The Coast Guard has dispatched aircraft to bring the crew to safety.
The crew decided to abandon ship after taking on water and losing communications and propulsion. The vessel was about 250km west of the eye of the hurricane.
Al Jazeera's Kristen Saloomey, reporting from New York, said the entire East Coast was preparing for the hurricane.
The US government has estimated the storm could affect 50 million people, and the storm has already been disrupting transportation systems.
More than 700 flights, both domestic and international, were cancelled on Sunday and about 2,500 more were cancelled for Monday, the website FlightAware.com said.
A special Reddit thread has been set up to share information and tips between internet users for the best ways to get through the storm.
Google is also hosting a live crisis map , featuring locations and capacities of emergency shelters, localised weather reports and satellite imagery.
Hybrid 'super storm'
Forecasters say Sandy is a rare, hybrid "super storm" created by an Arctic jet stream wrapping itself around a
tropical storm, possibly causing up to 30cm of rain in some areas, as well as heavy snowfall inland.
"The size of this alone, affecting a heavily populated area, is going to be history making," said Jeff Masters, a hurricane specialist who writes a blog on Weather Underground .
Harvard University, in Cambridge, Massachusetts, cancelled all classes in what was understood to be the first time in 40 years, and New Jersey casinos were ordered to close.
New Jersey officials have shut down the state's bus and rail systems, and emergency officials warned of widespread power outages that could last for days.
On its current projected track, Sandy is most likely to make landfall in the New York/New Jersey area and head inland to Philadelphia and Pennsylvania, forecasters said.
Forecasters at the National Hurricane Centre in Miami said the storm's impact would be felt far from the centre.
While Sandy's winds are not overwhelming for a hurricane, its width makes it exceptional. Hurricane-force winds extend 280km from its centre, while its lesser tropical storm-force winds span 1,670km in diameter.
It was not expected to strengthen, but was expected to broaden.
At high tide, the storm could bring a surge of seawater up to 3.4m above normal levels to Long Island Sound
and New York Harbour.
"Given the large wind field associated with Sandy, elevated water levels could span multiple tide cycles, resulting in repeated and extended periods of coastal and bayside flooding," the forecasters said.
Sandy killed at least 66 people as it made its way through the Caribbean islands, including 51 in Haiti, mostly from flash flooding and mudslides, according to authorities.
Click through our gallery for more photos, as millions of US residents brace for the storm: