Nearly a quarter of the US population fell under extreme heat advisories, partly due to a stubborn heat dome that has been parked over western states.
The heat warnings spread from the Pacific northwest, down through California, through the southwest and into the south and Florida.
Death Valley, California, officially reached 53C on Sunday, according to the National Weather Service, although the famed temperature display sign outside the Furnace Creek Visitors Center showed 56C.
Furnace Creek in Death Valley recorded the hottest recognised temperature ever on Earth at 56.7C in July 1913, according to the World Meteorological Organization.
Expecting to see history made, about two dozen tourists gathered around the sign and cheered on Sunday when the digital display ticked up to 56C. People snapped pictures while US National Park rangers stood by in case anyone succumbed to the heat.
"It's my first time being here so I feel it would be really cool to be here for the hottest day ever on Earth for my first time," said Kayla Hill, 24, of Salt Lake City, Utah.
Phoenix marked its 17th consecutive day over 43C and is forecast to tie the June 1974 record of 18 straight days over 37C on Monday and extend the record for at least another week.
The National Weather Service said widespread record-breaking high temperatures are likely to be recorded across the southwest, in the western Gulf Coast and also in south Florida.
Southern Europe is also enduring a punishing heatwave.
Italy has issued hot weather red alerts for 16 cities with meteorologists warning that temperatures will hit record highs in the coming days.
Spain, Italy and Greece have been experiencing scorching temperatures for several days already, damaging agriculture and leaving tourists scurrying for shade.
On the Spanish island of La Parma, at least 4000 people had to be evacuated as a forest fire burned out of control, authorities said.
A new anticyclone dubbed Charon, who in Greek mythology was the ferryman of the dead, pushed into the region from north Africa on Sunday and could lift temperatures above 45C in parts of Italy early this week.
"We need to prepare for a severe heat storm that, day after day, will blanket the whole country," Italian weather news service Meteo.it warned on Sunday.
"In some places ancient heat records will be broken."
In Spain, forecasters warned of the risk of forest fires and said that it would not be easy to sleep during the night, with temperatures unlikely to fall below 25C across the country.
The heatwave will intensify from Monday, with temperatures reaching 44C in the Guadalquivir valley near Seville.
Europe's highest recorded temperature of 48.8C, registered in Sicily two years ago, could be exceeded in the coming days, notably on the Italian island of Sardinia, meteorologists said.Â
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