Millions of people are still without power in the northeast United States and Canada in one of the largest blackouts ever.

A massive power grid failure late Thursday knocked out electricity to as many as 50 million people in New York, Ohio, Michigan, Pennsylvania and the Canadian cities of Ottawa and Toronto.

Electricity has been restored to a few parts of New York, but most of the city is still dark. Some buildings are being lit by emergency generators. City transit authorities say there will be no subway or commuter rail service Friday morning.

Some power is also back on in parts of Toronto, Canada. But reports say it may be several days before power is restored in Detroit, Michigan and the midwestern U.S. city of Cleveland, Ohio.

President Bush reassured Americans the blackout was not a terrorist act. The president said authorities are slowly but surely coping with this massive national problem, and he ordered a review of why such widespread outages occurred. Mr. Bush said the national electrical grid might need to be modernized.

Several U.S. states and Canadian provinces have declared a state of emergency.

The cause of the huge power failure remains unclear. U.S. and Canadian electric company officials seem to disagree on what caused the vast power-grid system to overload and fail.

At one point, Canadian power officials said they thought a lightning strike on a U.S. power plant started the blackout, but their U.S. counterparts ruled out lightning as a cause.

Regulators shut down nine U.S. nuclear power plants as the blackout hit.