With power supplies yet to return to much of northeastern United States and the Canadian province of Ontario, residents of New York, Detroit, Ottowa and a host of smaller cities spent the day in discomfiture at their homes.

Chaos and confusion meanwhile reigned in the streets outside. 

Many thousands were stranded and unable to get home, hundreds of flights were cancelled and commuter services at best remained skeletal.

Among all the overwhelming bad news, what perhaps brought some cheers to hapless residents was the government admission that the blackout wasn’t after all an act of terrorism.

Preliminary investigations suggested the crippling power outage that struck on Thursday was a result of a breakdown in the region's aging electricity infrastructure.

Preliminary investigations suggested the crippling power outage that struck on Thursday was a result of a breakdown in the region's aging electricity infrastructure.

New York Mayor, Michael Bloomberg signalled the gradual return of electricity, symbolically ringing the opening bell on the New York Stock Exchange. But he cautioned that the recovery would be slow.

"Power won't be restored to all New Yorkers until later today," Bloomberg said.

"I would not expect you to have subway service for the evening rush hour," he added.

Groping as much in the dark, officials still searched to pinpoint the exact cause of the outage.

US power grid operations called it the biggest power failure in North American history, worse than those of November 1965 and July 1977.

Stung by the blackout's cascading impact, the US House of Representatives' Energy and Commerce Committee announced it had launched an investigation into the reasons behind it.

A far immediate impact was felt by many hospitals, deluged by peoople with complaints of heat exhaustion and heat related ailments.

Bloomberg said a 40-year old woman died of a heat-related heart attack while evacuating from a building.

The New York Mayor told reporters that electricity was gradually returning to the city of eight million. Power was restored in Times Square but none of the "super signs" were still working.