A massive power blackout retreated stubbornly Friday as power officials struggled to understand why the historic outage spread in minutes through the northeastern United States and southern Canada.

Lights flickered on and air conditioners restarted for some, but millions of others baked in stuffy rooms.

Cleveland faced the worst water crisis in its history as the blackout shut down all four major pumping stations for the first time ever.

In New York City, where lights began to flicker on in parts of midtown Manhattan and other boroughs and suburbs before dawn Friday, millions faced a morning rush hour without subway service and no timetable for full restoration of power. In Michigan, some customers may have to endure a weekend without electricity. Everywhere, officials urged residents, businesses and travelers to cope with the inconvenience.

''This is truly one of the instances where we're all in this together,'' Gov. Jennifer Granholm of Michigan said during a statewide address Thursday night. ''So be calm, be supportive of your neighbor.'' State workers in Michigan's capitol, Lansing, were told to report to work Friday, but in harder-hit Detroit to the east, they were ordered to stay home.

While terrorism was swiftly ruled out by President Bush and other officials, there was scant indication of what had caused the outage, which began on the cusp of Thursday's afternoon rush hour in Eastern cities.

The New York Independent System Operator, which runs the state's wholesale electricity market and monitors power usage, said it had detected a sudden loss of power generation at 4:11 p.m.

Kenneth Klapp, an ISO spokesman, said the problem was detected from information on power usage and transmission prior to, during and after the blackout. The ISO had not determined the exact location of the problem by early Friday.

More generally, industry and government experts blamed a system composed of interconnected grids that has not been upgraded to meet power demands.

The disruptions were as diverse as they were widespread.

A small explosion at the Marathon Oil refinery 10 miles south of Detroit was blamed on the outage which cut power to a pump, allowing a buildup of gasses that ultimately exploded in a smokestack. No one was hurt but police fearing additional explosions or possible release of toxic gas evacuated hundreds of residents from a one-mile radius around the refinery.

In New York City, thousands of stranded commuters were forced to sleep in bus and train terminals.

and even in the streets. Hundreds of out-of-towners at the Marriott Marquis slept on sidewalks because the hotel did not have a generator to power its electronic room keys.

In Cleveland, the loss of power wasn't the only problem. About 1.5 million residents faced a crisis because there was no electricity to pump water from Lake Erie. At least three Eastern suburbs were out of water and officials said Western suburbs could go dry.

About 540,000 customers in Ohio were without power, mostly in the Cleveland area.

In New Jersey, where more than 1 million homes and businesses lost power at the peak of the outage, all but 50,000 had been restored by 5:30 a.m. Friday and full service was expected a few hours later. Northern New Jersey commuter railroads and buses announced limited to full service Friday.

In Connecticut, where nearly 310,000 customers served by two power companies lost power, all but about 53,000 had service restored by early Friday.

But in New York, where early estimates had 80 percent of the state without power, the percentage only dropped to some 60 percent near midnight.

Despite the outages in Manhattan, New York's financial markets had no intention of shutting down.

The American Stock Exchange, New York Stock Exchange and Nasdaq reported minimal interruption after the close of trading. All had backup power generators and said they planned to open Friday.

However, businesses from Manhattan through the Midwest were anxious about technical glitches and more power outages a day after the biggest blackout in U.S. history.

In San Diego, the president said, ''slowly but surely we're coping with this massive, national problem,'' and added that he would order a review of ''why the cascade was so significant.''

Bush said he suspected that the nation's electrical grid would need to be modernized.

New York Gov. George Pataki praised his constituents for pulling together to help each other. While New Yorkers poured out of immobile subway cars, emerged from stuck elevators, began long walks home or rested in local establishments, one unidentified man saw beauty.

''You can actually see the stars in New York City,'' he said.

Anne Block, a law student in Lansing, Mich., said she used what little light was coming through a window to finish an exam at Thomas M. Cooley Law School.

''We were taking an exam and boom, the lights went out. But I was determined to finish. I kept writing. I wanted an 'A.' There was no way I was going to stop writing my exam,'' she said.

Electric industry and government officials said the nation's power grid has needed major upgrades for years, but industry experts said there were three major obstacles in the way: the expense, environmental opposition and people who didn't want power facilities near their back yards.

Both federal and state agencies, as well as congressional committees, are expected to investigate the blackout and try to determine why measures put in place to isolate grids and keep power disruptions from spreading failed to do so.

Law enforcement agencies were ready for any security problem.

In New York, police helicopters, boats and heavily armed teams of special counterterror officers moved into place at city landmarks and other sensitive locations, police Commissioner Ray Kelly said. Officials swiftly realized the power outage was not an act of terror, and they used so-called Atlas teams of officers to make sure no one took advantage of the blackout to commit terrorism, he said.

Officials in Detroit urged people to stay home during the night; nearby communities declared curfews to keep problems to a minimum.

Police in Mansfield, Ohio, spread into the streets to keep traffic flowing. ''A lot of officers are out there trying to make sure nobody gets hurt, to try to cut down on the accidents,'' said jail officer Randi Allen.

The blackouts easily surpassed those in the West on Aug. 10, 1996, in terms of people affected. Then, heat, sagging power lines and unusually high demand for electricity caused an outage for 4 million customers in nine states.

An outage in New York City in 1977 left 9 million people without electricity for up to 25 hours. In 1965, about 25 million people across New York state and most of New England lost electricity for a day.