A record-breaking heat wave tightened its grip on New York City on Tuesday, as triple-digit temperatures tested Consolidated Edison's power supply, threatened the health of the elderly, and tried the patience and resilience of anyone who dared to venture outside.

With the temperature reaching 103 degrees in Central Park at 3:11 p.m., breaking the former record high of 101 degrees for the day set in 1999, Con Edison officials braced for the greatest demand for power they had ever had to supply. The long red arrow on the dial projected on a screen in the utility company's command center in Manhattan hovered at the threshold of uncharted territory -- 13,141 megawatts consumed at one time -- for most of the afternoon.

The heat broke several records in the Northeast, as Boston, Providence and Philadelphia all saw temperatures in the 100s that eclipsed previous highs. In Philadelphia, a 92-year-old woman was found dead in her home on the second floor. The medical examiner ruled that extreme heat was a factor in her death.

The National Weather Service blamed a high-pressure system along the East Coast that drew hot, humid air from the south and will probably stay put until late this week.

In New York, the heat's effects were unsparing: Some city pools were filled to capacity within an hour or so of opening, sending seekers of respite to libraries, cooling centers and other public havens from the heat. Hospitals set out jars of ice water as their waiting rooms filled with wheezing elderly patients and exhausted firefighters.

Police Commissioner Raymond W. Kelly said that the Police Department was mindful that crime sometimes increased in the extreme heat of night, and was also prepared to send extra officers to places that had lost power.

But even as Con Edison officials were optimistic that the city would survive the day without widespread power failures, they acknowledged that the intensity and duration of the heat wave could have a cumulative effect on the cables and transformers. In short, they said, the worst may be yet to come. ''It's Round 1 in a prizefight,'' said John Miksad, Con Edison's senior vice president of electric operations. ''Round 1 looks O.K., but the bell hasn't rung yet.''

It has been four years since the utility's equipment in Long Island City failed in a cascade of blown feeder lines and left tens of thousands of Queens residents without power for more than a week. The power system's ability to withstand a sustained surge in demand has not really been tested since that summer.

''We haven't had a real New York heat wave in a while,'' said Mr. Miksad, who admitted to having fretted through a ''nervous weekend'' knowing what was coming. Con Edison dispatched extra crews to Staten Island where a main feeder cable failed early in the day, and by late afternoon more than 4,000 customers were without power in the Fox Hills neighborhood.

By late Tuesday evening, Con Edison had reduced voltage in neighborhoods throughout Brooklyn and Queens because of problems with electrical cables, and customers were receiving automated calls asking them to turn off ''nonessential'' appliances. And in New Jersey, 24,000 customers across the state were still without power at 10 p.m., said the Public Service Electric and Gas Company.

The power failures came as city officials and utility executives dealt with what was the hottest day in New York since Aug. 9, 2001, when the temperature in Central Park also reached 103 degrees, according to the National Weather Service.

But Mr. Miksad said that more people might have lost power if not for a set of demand-reduction programs that were used on Tuesday, including voluntary cutbacks by big corporate customers and the utility's ability to control the thermostats in some residents' homes. All told, those programs saved as much as 400 megawatts and kept total demand from surpassing the all-time high, he said.

Some office buildings, including Con Edison's headquarters near Union Square, shut down banks of elevators, lowered the lights and turned up thermostats. Con Edison requested that all of its customers conserve electricity by turning off equipment not being used, keeping air-conditioners at 78 degrees and running washers, dryers and dishwashers late at night.

Robert Madden, 25, a waiter who lives in Astoria, said that his electronic equipment shut down in a specific order.

''First the PlayStation turns off, then the refrigerator, then the computer, the lights, then the pilot lights on the stove, then the fan,'' he said.

By late afternoon on Tuesday, New York-Presbyterian Hospital/Weill-Cornell Medical Center had seen only scattered cases of heat-related illnesses, but doctors there said that the numbers would most likely increase.

''Yesterday was much easier than today, which will be much easier than tomorrow'' as the high pressure keeps the air from circulating, saturating it with pollutants, Dr. Neal E. Flomenbaum, the hospital's emergency physician-in-chief, said on Tuesday.

At Elmhurst Hospital in Queens, 17 firefighters were treated for heat exhaustion after a three-alarm blaze at a two-story house in nearby Astoria in the morning, a hospital spokesman, Dario Centorcelli, said.

At Barretto Point Park in the Hunts Point neighborhood of the Bronx, the floating pool reached capacity at 12:10 p.m., barely an hour after it opened, a parks department spokesman said. After that, people were allowed in only as others left.

Several other public pools reached capacity -- at 1:30 p.m. at John Mullaly Park in the Highbridge section of the Bronx and at 2:30 p.m. at Sunset Park in Brooklyn, for example -- and beach attendance made it seem as if the holiday weekend had been extended by a day. About 270,000 people went to city beaches, most of them to Coney Island, where parks employees estimated the crowd to be around 205,000, the parks spokesman said. That was triple the number who went to the beaches on July 6 last year, he said.

On a small bench at the back of the Harlem branch of the New York Public Library, Frankie Santiago, 15, clutched his iPod and shared his headphones with Carmen Rivera, 14. ''We were going to go to the pool,'' said Frankie, who lives in Harlem. ''But it was closed, so we thought it was better to wait here than in the heat.''

Nearby, Charlene Williams, 60 and unemployed, waited for her turn at a 45-minute session on one of the computers in the library, which is on 124th Street opposite Marcus Garvey Park.

''If they allowed me two-hour sessions, I would stay that long to stay out of the heat,'' she said with a laugh.

The lack of air-conditioning in some dormitories at New York University had renters questioning their decision to spend the summer in the city.

Johnnie Kling, 23, moved to New York from Ohio last week to find a job. But he was finding it difficult to remain presentable.

''Sometimes I'll duck into a retail store just so I can acclimate and not have beads of sweat on my forehead during an interview,'' Mr. Kling said.

PHOTOS: Several hundred people gathered at the entrance to Highbridge Pool in Washington Heights on the year's hottest day. (PHOTOGRAPH BY OZIER MUHAMMAD/THE NEW YORK TIMES); Yvonne Hicks, 70, shopped for an air-conditioner in Downtown Brooklyn. (PHOTOGRAPH BY PIOTR REDLINSKI FOR THE NEW YORK TIMES) (A19)