The eastern United States wilted Tuesday in a heat wave expected to last all week, putting power companies under pressure and driving city residents into municipal "cooling centers" -- or fountains.

Accuweather.com forecasters said temperatures already exceeded 100 degrees Fahrenheit (38 degrees Celsius) in New York City, Washington, DC, and Richmond, Virginia, and would soon match that in Philadelphia.

This was the first time central New York had seen triple digit heat in almost nine years, Accuweather.com said.

Similar conditions stretched as far north as Montreal in Canada's Quebec region.

New York state Governor David Paterson said "a significant heat wave... is forecasted for this week, particularly in New York City and on Long Island, where conditions are expected to be at their worst."

Electricity use surged through the densely populated eastern seaboard of the United States region as office buildings reopened after the long July 4th Independence Day weekend.

Con Ed, a major utility company in the New York area, urged energy conservation during what was expected to be record demand for power, with customers putting air conditioners on overdrive and raiding freezers and fridges for refreshments.

Paterson recommended New Yorkers reduce electricity use during the peak hours of 1:00 and 7:00 pm.

The National Weather Service issued a heat advisory, saying that heat and high humidity could prove dangerous for young children and the elderly.

The NWS warned residents to "drink plenty of fluids, stay in an air-conditioned room, stay out of the sun and check up on relatives and neighbors."

"Excessive heat" advisories also were posted in New York City and Philadelphia, where authorities opened "cooling stations" -- air-conditioned rooms in schools and other neighborhood buildings where residents could get out of the cauldron.

New York's health and emergency management departments issued heat tips including keeping "strenuous activity" to between 4:00 and 7:00 am.

The city government suggested residents without air conditioning at home either visit cooling centers or spend the day in the "mall, museum or movie theater."

For millions of children on summer vacation, city fountains and the time-honored option of fire hydrants provided instant relief.

In a bid to prevent the illegal opening of hydrants, New York fire houses were issuing spray caps designed to keep the gush to a manageable level.

Heather Buchman, a meteorologist for AccuWeather.com, warned on the website of "a dangerous heat wave" both for people and infrastructure.

"In addition to putting stress on people and their health, the intense heat will also place a higher demand on power grids due increased usage of air conditioning. Power outages could result in some communities, putting people at an even greater risk for developing heat-related illnesses," she said.

In August 2003 the entire US northeast and part of Canada were plunged into darkness when the power grid collapsed in one of the biggest outages in history.