Nearly 150 million people across the country are facing hazardous temperatures in a heatwave forecast to stretch from the Midwestern plains to the Atlantic coast, the National Weather Service said Friday.

Heat warnings have also been issued in parts of eastern Canada.

New York City has opened 500 cooling centers for residents to escape the extreme weather.

"Saturday is going to be really, really bad, on through Sunday," said Mayor Bill de Blasio in a warning to the city.

"Take care of yourself, don't go out in the heat if you don't need to... This is serious, serious stuff."

Climate data showed June was the hottest month on record worldwide, with a heatwave across Europe smashing national temperature records.

June temperatures across the continent were 3C hotter than the baseline average between 1850-1900, the European Union's Copernicus Climate Change Service found.

Temperatures soared earlier this month in the northwestern US state of Alaska, which straddles the Arctic Circle, with largest city Anchorage hitting an all-time high of 90F.

According to scientists, Alaska is warming at twice the rate of the global average.