A powerful winter storm pummeled northeastern United States on Monday, unleashing blizzard conditions that blanketed major cities in snow, paralyzed travel, closed schools and caused massive power outages.

Around 65 million people, about 20 percent of the U.S. population, were placed on alert, with about 35 million people under blizzard warnings stretching from Maine to coastal Virginia.

The storm severely snarled air and ground travel. Thousands of flights were canceled nationwide, with New York-area airports hit the hardest.

As of 9 p.m. on Monday, more than 5,700 flights within, into or out of the U.S. had been canceled and over 3,400 delayed, according to flight-tracking website FlightAware. John F. Kennedy International Airport in New York reported the highest number of cancellations, followed by LaGuardia Airport and Newark International Airport.

Rhode Island's T.F. Green International Airport halted all flights Monday as it dealt with nearly 97 centimeters of snow, according to the Weather Service.

Over half a million homes and businesses across the northeast were without power Monday afternoon, according to PowerOutage.us.

Massachusetts topped the list, with nearly 300,000 users experiencing outages. More than 100,000 remained without power in Cape Cod, Massachusetts, which operates essentially on a single circuit.

Other states facing major outages included New Jersey, Delaware, Rhode Island, New York, Maryland and Connecticut.

New York City received 41-48 cm of snow as of Monday afternoon, New York City Mayor Zohran Mamdani said at a news conference. Parts of eastern Staten Island reported more than 60 cm of snow.

A travel advisory is in place until midnight, the mayor said, urging New Yorkers to stay home as accumulated snowfall and refreezing conditions have made it dangerous to travel.

Officials warned that the storm could rank among the city's 10 worst in 150 years, which also forced the closure of the UN headquarters complex in Manhattan.

At least seven states – New York, New Jersey, Pennsylvania, Delaware, Connecticut, Rhode Island and Massachusetts – have declared state of emergency in response to the severe weather. Some have imposed commercial vehicle bans and travel restrictions.

New York Governor Kathy Hochul said she had activated 100 National Guard members to assist in Long Island, New York City and the lower Hudson Valley, areas expected to bear the brunt of the storm.

In Massachusetts, 200 National Guard members have been deployed for storm recovery assistance, Governor Maura Healey said. A state of emergency remains in effect.

Train and bus commuter lines in New Jersey were halted, while the Rhode Island Public Transit Authority suspended all service from Sunday night through Monday.

In an update on the winter storm, Connecticut Emergency Management Director William Turner said that more outages were expected, urging people to stay off the roads.

The state was still under a state of emergency on Monday afternoon, with a commercial travel ban in effect. Severe cold-weather protocol will remain in place until at least Tuesday noon, according to Turner.