Hurricane Irene has been downgraded to a tropical storm, but continues to pound the U.S. east coast with heavy rains and strong winds as it moves over New York City.

The storm made landfall in North Carolina Saturday morning. It has killed at least eight people and paralyzed ground and air traffic in its journey up the eastern U.S. coast.

The National Hurricane Center says Irene is approaching New York with winds of 120 kilometers per hour. Forecasters say the upper floors of the city's many high-rise buildings will experience significantly stronger winds than at ground level. Water levels are expected to rise as much as two and a-half meters, posing a problem for Manhattan's low-lying areas such as Battery Park, on the tip of the island.

Part of an underground traffic tunnel leading into the city has been closed because of flooding. The East and Hudson rivers surrounding the island of Manhattan have overflowed and some streets are flooded.

New Jersey Governor Chris Christie has predicted record-level flooding for parts of his coastal state.

Forecasters say Irene will move into southern New England by Sunday afternoon before reaching Canada Sunday night.

Effects of the storm have forced two nuclear reactors to cease operations – one in Maryland and one in New Jersey. Authorities say the facilities are safe, and no one has been harmed.

Irene blasted ashore in North Carolina early Saturday, flooding streets and toppling trees with winds of 140 kilometer per hour. The storm later moved into the Washington, DC area, which was hit with strong winds, heavy rain, localized flooding and falling trees.

About 2 million homes and businesses are without power in the eastern U.S. Tens of millions of people are in the path of the storm, which is passing through some of the country's most densely populated areas. Suspected tornadoes spurned by the hurricane destroyed homes in Delaware and Virginia.

The Hurricane Center warns of life-threatening waves and surf along the coast, with a storm surge raising water levels more than 2 meters in some areas. Rainfall of 25 centimeters or more could cause life-threatening flash floods and significant uprooting of trees. Many trees already have fallen where Irene has passed.

On Saturday, President Barack Obama visited the command center where senior emergency officials have been coordinating the country's response. He warned the situation would be critical for the next few days.

The U.S. Navy ordered the Second Fleet, stationed in Virginia, out to sea to avoid potential damage.

In New York, Mayor Michael Bloomberg ordered the city's first ever mandatory evacuation 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 have been shut down, as well as all three of New York's major airports.

Airlines have already canceled thousands of flights up and down the east coast, and train service in parts of the eastern region have been suspended.

The Red Cross said its response to Irene could be one of the largest it has undertaken in recent memory. The organization is responding to more than a dozen states, and says it could take weeks, even months, to be able to fully address the disaster.

Irene was the first hurricane to seriously threaten the United States in three years. Besides the U.S. fatalities, Irene killed at least one person in Puerto Rico and two in the Dominican Republic, and destroyed homes in The Bahamas.

It came almost six years to the day after Hurricane Katrina flooded New Orleans, killing more than 1,800 people and forcing more than a million residents from their homes.