Temperatures hit or hovered near 100 degrees Fahrenheit in New York, Philadelphia and Washington, with hotter weather forecast for Wednesday. Detroit, St. Louis and Chicago were sweating it out in equally stifling temperatures.

"It is miserably hot outside and hard on everyone," New York Mayor Michael Bloomberg told reporters. "This is a very dangerous heatwave. It's really more than just uncomfortable, it can seriously threaten your lives."

The National Weather Service issued excessive heat warnings and said the heat index -- how hot it feels when the humidity is combined with the air temperature -- was due to hit 115 F (46 C) in New York on Wednesday.

"If people do not take precautions, we could be looking at a significant number of fatalities," said Gary Conte, the weather service's warning coordinator meteorologist, adding that New York City had not suffered such a string of high temperatures since July 1999.

"The forecasted temperatures and heat indices (in 1999) were pretty close to what we're looking at now. The impact from that event resulted in 43 deaths in New York City and New Jersey with rolling blackouts, buckled roads and so forth."

RECORDS BROKEN

The National Weather Service said more than 50 temperature records had been set in the central and western United States in the past two weeks.

"The persistence of the unusually hot temperatures has made the past month one of the warmest since records began in 1895 for the contiguous US," it said.

Meteorologists are analyzing data to determine if July 2006 has surpassed July 1936 to become the hottest on record.

New York City has opened hundreds of air-conditioned "cooling centers" and extended hours at public swimming pools, while urging the public not to open fire hydrants.

"It's too hot. It's hard to work, but we have to suffer to make a living," said Tajdar Sayed, who has been selling fruit from a street stand near New York's Times Square for 15 years.

Electricity grid operators did not expect to have to impose rolling blackouts, aimed at preventing uncontrolled outages, due to any lack of generating capacity.

However, in some regions, power distribution cables could fail, like those that recently left 25,000 Con Edison customers in New York without power for up to a week.

ConEd said late on Tuesday it had set a new record for peak electricity usage, reaching 13,103 megawatts at 5 p.m., which topped the previous record of 13,059 MW set on July 27, 2005.

In 2003, the worst blackout in North American history left up to 50 million people in Ontario, Canada, and eight US states in the dark.

Commonwealth Edison reported about 10,000 scattered outages on Tuesday across its Illinois territory, including 2,700 customers on the south side of Chicago, who lost power Monday when an underground cable failed, spokesman Tom Stevens said.

In El Paso, Texas, heavy rains temporarily broke the region's drought and turned streets into raging rivers that uprooted trees and carried away cars.