Millions of people on the East Coast of the United States faced power outages and flooding Monday after Hurricane Irene weakened and moved into Canada.

Authorities reported at least 21 storm-related deaths in several eastern states, with damage amounts in the billions of dollars. U.S. President Barack Obama said Sunday that recovery efforts will take weeks or longer, and he promised federal help for state and local governments.

Forecasters downgraded Irene to a post-tropical storm early Monday as it moved over eastern Canada, bringing strong wind and rain to the region. It weakened from a hurricane into a tropical storm on Sunday before crossing over the northeastern United States, but still triggered major flooding in the states of New Jersey and Vermont.

The eye of the storm passed over New York City but spared the city major damage. Local authorities ordered an unprecedented shutdown of its subway system on Saturday ahead of Irene's arrival, depriving millions of their main mode of transportation. But city officials credited the shutdown with allowing them to re-open the subway system in time for Monday morning's rush hour.

But trains connecting New York to its northern suburbs and most rail services in New Jersey remained suspended due to flooding.

The New York Stock Exchange began its trading week on time Monday.

Irene first hit the United States on Friday, making landfall in the state of North Carolina, before moving along the mid-Atlantic coast toward New England.