Hurricane Irene stormed ashore in the eastern U.S. state of North Carolina Saturday, beginning its potentially devastating trek up the East Coast.

Irene carried winds of 140 kilometers per hour as it made landfall and battered the eastern U.S. coast with heavy rains.

The storm has already toppled trees, flooded streets and knocked out power to tens of thousands of people.

Forecasters at the National Hurricane Center downgraded Irene to the weakest category for hurricanes with winds at 150 kilometers per hour in the early morning hours of Saturday. But they still cautioned that the storm would remain a hurricane as it moves into the heavily populated corridor around Washington.

A hurricane warning has been issued for New York and much of the eastern U.S. coast in what. President Barack Obama is calling “an extremely dangerous and costly” storm.

In New York, Mayor Michael Bloomberg has ordered the city's first ever mandatory evacuation. Some quarter-million people have been told to leave their homes in low-lying areas of the city, including the Wall Street financial district. And in another first, New York's entire public transit system, including subway trains and buses, will shut down Saturday. All three of New York's major airports will shut down starting at mid-day as well .

Airlines have already canceled hundreds of flights elsewhere on the east coast, and train services in parts of the eastern region have been suspended.

Irene is the first hurricane to seriously threaten the United States in three years. It has already killed at least one person in Puerto Rico and two in the Dominican Republic, and also destroyed homes in The Bahamas.

It also hits at the six-year anniversary of Hurricane Katrina, whose flooding killed more than 1,800 people and forced more than a million residents from their homes in the southeastern United States. The federal response to that disaster was widely criticized as slow and mismanaged, and U.S. officials are determined to be prepared this time.

Mr. Obama says all indications point to Irene being a “historic” storm. He said the nation has to be “prepared for the worst.” He returned to Washington late Friday, leaving his vacation in Massachusetts a day earlier than planned. President Obama has directed agencies to ensure all the needed resources are available.

Homeland Security Secretary Janet Napolitano said Friday that federal and local authorities are taking the storm “very seriously.”

The threat of the hurricane also led organizers in Washington to postpone Saturday's dedication of a memorial to civil rights leader Martin Luther King, Jr.