Blizzards are sweeping north along the eastern coast of the US and Canada, forcing the cancellation of flights and disrupting rail and road traffic.
The storm closed some airports for hours, stranding thousands of people in the busy post-Christmas travel period.
The New York area was in the storm's bull's eye, receiving up to 51cm (20in) of snow, and even more in some areas.
Massachusetts, Maine, Maryland, New Jersey, North Carolina and Virginia declared emergencies.
The storm reached Canada's Atlantic coast early on Monday. Around 27,000 homes in Nova Scotia and 11,000 consumers in the New Brunswick area were left without power, according to reports.
Blizzard conditions are buffeting New Brunswick, where winds were gusting up to 100km (62mi) per hour.
The US National Weather Service said the monster snow storm was the result of a low pressure system which originated off North Carolina.
The full-blown blizzard - meaning winds of at least 35mph (55km/h) along with snow and low visibility - intensified near Cape Cod, Massachusetts before pummelling southern New England and the New York City area.
Blizzard warnings remained in effect for coasts of the northern mid-Atlantic states as far as Maine. More snow was expected in Boston and New York on Monday afternoon.
The southern states of Georgia and South Carolina had their first white Christmas in more than a century.
But Washington DC escaped the blizzard, with only a dusting of snow.
Forecasters later said they expected milder weather for the rest of the week, which could help in speeding up the clearing of snow.
The timing of the snowstorm meant disruption for many thousands travelling after Christmas reunions and hampered the start of the shopping sales season and the return to work for many commuters.
US officials say that more than 2,000 flights have been cancelled.
Three airports serving New York - JFK, La Guardia and Newark Liberty International Airport - and also Boston's Logan airport closed on Monday morning. Many passengers had to camp out on floors in terminals.
All four airports finally reopened in the evening, but officials warned that it could take until next week to clear the flights backlog.
Announcing a state of emergency in Massachusetts, Governor Deval Patrick warned that the storm was "expected to produce widespread heavy snowfall, periods of zero visibility, high winds, power outages, coastal flooding, and beach erosion", reported AFP news agency.
Power had already reportedly been cut to tens of thousands of homes in Rhode Island and Massachusetts.
In New York City, some 2,400 street cleaners were working 12-hour shifts to clear the snow from the city's 6,000 miles of roads - but inhabitants were advised to stay at home anyway.
"I understand that a lot of families need to get home after a weekend away, but please don't get on the roads unless you absolutely have to," said Mayor Michael Bloomberg.
Amtrak announced it was cancelling rail services from New York to Maine as well as in Virginia, after earlier doing the same for New York City-Boston services.
New York's Long Island Rail Road remained suspended and many bus routes serving the east coast were also cancelled.
The conditions were blamed for a car crash in Maine in which a 59-year-old man died, and for stranding two buses carrying some 50 passengers on a New Jersey motorway. State troopers carried water and food to some of the passengers who are diabetic.
Stranded cars were hampering the efforts of snow ploughs and ambulances, but passengers on one bus have been rescued and an operation was under way to free passengers aboard the other.