Power was filtering back to northeastern US and southern Canada after the largest blackout in American history affected some 50 million people, leading to the evacuation of office buildings and stranding thousands of commuters.

"We have begun a careful restoration. This has to be done methodically, and we are urging people to turn off their
appliances as the power comes back so we don't get a surge in the system," a spokesman for Consolidated Edison, which supplies power to 3.1 million customers said.

Circuit breakers had earlier tripped at generating stations from New York to Michigan and into Canada leaving millions of people in the dark besides affecting rail and air communication.

Officials said the cause of the blackout was under investigation but terrorism did not appear to have played a role.

The White House said there was no indication that terrorism was involved.

"It's a serious situation. I have been working with federal officials to make sure the response to this situation was quick and I believe it has been," President George W Bush told reporters in San Diego.

Bush also made an appearence on TV to congratulate the people for the way they faced the problem.

New York Mayor Michael Bloomberg said: "There is no evidence whatsoever of terrorism. We will be starting up power in the city. It will take a decent amount of time, hours not minutes - and nobody can be any more specific than that."

Authorities, however, were maintaining strict vigil to rule out any untoward incident.

US officials were looking at a power transmission problem from Canada as the most likely cause of the outage.

The office of the Canadian Prime Minister Jean Chretien initially said the power problems were caused by lightning in New York State but later retracted the statement, the New York Times reported.

Canadian officials later expressed uncertainity about the exact cause but insisted the problem began on the US side
of the border.

The power outage threw train and air transport out of gear stranding thousands of commuters.

Airports throughout the affected states suffered serious disruptions, including the three major airports in the New York metropolitan region, but did not close, the Times reported.

However, delays and cancellations rippled all the way to San Francisco. Federal aviation officials said the airports in the affected states had switched to emergency power.
 
Trains stopped in their tracks between stations and in tunnels. Hundreds of people were guided tto the next station and safely pulled out onto the platform.

With no trains and buses on several streets, many commuters walked miles to reach homes.

People were also caught between floors in elevators.