A massive power blackout has struck the northeastern United States and eastern Canada as the region sweltered in August heat. Government officials have reassured a nervous public that terrorists are not responsible for the power loss.

The blackout shut down nuclear power plants in New York state and Ohio and trapped New York city dwellers in subways and others in the region in elevators. It drove New York workers out of office buildings into the streets, many of them forced into a long walk home in blistering heat. Traffic lights went out in New York, Cleveland, Detroit and other cities just as afternoon rush hour began, creating havoc on the streets. New York police dispersed throughout the city to direct traffic.

Air traffic also slowed as aviation authorities halted flights to airports in New York, Cleveland, Ottawa, and Toronto.

To allay fears, New York Mayor Michael Bloomberg used understatement to call the blackout a major inconvenience, apparently a technical failure of the power grid.

"There is no evidence of any terrorism whatsoever," he said. "For some reason or other, there was a power failure in northern New York or southern Canada. That cascaded down through the system and affected the power grid as far east as Connecticut, as far south as New Jersey, and as far west as Ohio."

The confirmation that terrorism was absent has come from U.S. national authorities, including the U.S. Department of Homeland Security, a new cabinet agency created after the 2001 terrorist attacks on New York and Washington.

The governor of the state of New York, George Pataki, declared an emergency, but Mayor Bloomberg says the people of his city remained calm and suffered no injuries as they reacted to the massive power loss.

"People are doing what you would expect them to do in New York City they are cooperating," he said. "Everyone has been as helpful as you could possibly ask them to be. With a lot of luck, we will look back on this and say, 'Where were you when the lights went out,' but nobody will have gotten hurt."

This is not the first northeastern U.S. power blackout, but it rivals previous ones in scope. Others occurred in 1965, 1977, and 1996. The last one seven years ago was one of the most severe, covering nine states after heat, sagging power lines, and unusually high power demand shut down the grid. The 1977 blackout lasted a full day.