Rare October snows transformed the northeastern United States on Sunday into a Halloween winter wonderland, but millions were left without power and three deaths were blamed on the storm.

The unseasonable snowfall disrupted air, rail and road traffic from Washington to Boston and the National Weather Service warned of 'extremely hazardous' conditions as states of emergency were declared across the region.

Three deaths were reported: a man was killed when he touched an electrified guardrail in Massachusetts; a motorist crashed fatally on icy roads in Connecticut, and a Pennsylvania man was killed when a tree fell on his home.

As dawn broke with Americans gearing up for Monday's Halloween celebrations, the extent of the snowfall was revealed - up to a stunning 31 inches (78 centimeters) in the New Hampshire town of Jaffrey.

In New York's Central Park, where experts said there hadn't been an inch of snow on an October day since records began in 1869, there was an unprecedented 2.9 inches.

From Washington in the south to the towns of northern Massachusetts, there were strangely wintry scenes for an American Halloween.

Ghosts and ghouls on front lawns and porches were shrouded in snow. Families stocking up on candy, prepared for some particularly chilly trick-or-treating with shovels and salt at the ready.

There was misery for some as energy companies reported up to three million homes without power. Worst hit were Massachusetts (660,000 customers), Connecticut (740,000), and New Jersey (500,000).

Connecticut Light & Power described the damage as 'unprecedented' and warned residents to prepare for a worst-case scenario of a week or more without power.

Air travelers who had seen an average delay of six hours on Saturday were faring better as major airports in New Jersey, New York and Massachusetts reported flight schedules returning to normal.

Rail travel had also been hit, with multiple Amtrak services remaining canceled along the busy northeast corridors due to signal problems caused by the storm.

Passengers on one train in rural Vermont were trapped overnight after it hit a tree on the line at around 4:00 pm (2000 GMT) on Saturday. They were finally allowed off, some 20 hours later, at midday (1600 GMT) on Sunday.

An average of a foot (30 centimeters) of snow fell in parts of Pennsylvania, Connecticut and New Jersey.

Unseasonably cold air was pouring into the northeast, and deep tropical moisture was set to surge northward along the east coast and 'fuel an expanding area of heavy rain and snow', the National Weather Service said.

Much of the region was hit by Hurricane Irene in August. Its heavy rains and wind killed more than 40 people, left millions without power, destroyed homes and caused record flooding.

The unseasonably cold and wet weather did not dampen the spirits of anti-Wall Street protesters camped out in New York and Washington.

"Snow, what snow? I've got a country to worry about," read a sign held by a woman at New York's Zuccotti Park - the nerve center of the Occupy Wall Street movement.

At the White House, President Barack Obama and wife Michelle did not let the weather ruin their annual Halloween trick-or-treating event, handing out candy, cookies and dried fruit to children wrapped up in thick coats.