The United States was sweating through a weekend of dangerously hot weather, with major cities including New York, Philadelphia and Washington experiencing temperatures approaching 38 degrees Celsius.
An oppressive heat wave stretching from the Midwestern plains to the Atlantic coast had nearly 150 million people struggling to stay cool amid scorching temperatures.
Heat index values -- combining the effect of heat and humidity -- could reach 110 to 115 degrees, particularly in the east, the National Weather Service (NWS) said.
It warned that "dangerously high temperatures and humidity could quickly cause heat stress or heat stroke if precautions are not taken."
"Saturday is going to be really, really bad, on through Sunday," de Blasio said in a warning to the city. "This is serious, serious stuff."
At least three public defenders said on Twitter that inmates in New York's notorious Rikers Island jail complex were suffering with no air conditioning, and that some guards had turned off fans as punishment, resulting in "deadly conditions."
Those in units without air conditioning were given access to fans, ice, water and "multiple cool showers."
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