Across vast swathes of the planet, from California to China, authorities have warned of the health dangers of the extreme heat, urging people to drink water and shelter from the burning sun.
In a stark reminder of the effects of global warming, the UN's World Meteorological Agency (WMO) said the world should prepare for increasingly intense heat.
"These events will continue to grow in intensity, and the world needs to prepare for more intense heatwaves," John Nairn, a senior extreme heat advisor at the WMO told reporters in Geneva.
The peak in Europe was due to hit in the Italian islands of Sardinia and Sicily, where temperatures could surpass the continent-wide record of 48.8 degrees Celsius (119.8 degrees Fahrenheit) recorded in Sicily in August 2021.
In Rome, where temperatures were expected to reach 42C, two priests ambled lethargically with their collars open outside the Pantheon as local merchants filled up water buckets at public fountains.
"We can't stop because of the heat," said waiter Mauro Natale, 45, who said staff was allowed to wear short-sleeved shirts instead of their usual suits.
Tourists still needed to eat, but "instead of a carbonara or an amatriciana (pasta), they just want to eat salad."
Northwest of the Greek capital Athens, a vast cloud of smoke loomed over the forest of Dervenohoria, as emergency services battled wildfires for a second day in several locations around the capital.
"Our main concern is protecting human life," firefighters spokesman Yannis Artopios told the press.
Still burning was a forest fire by the popular beach town of Loutraki, where the mayor said 1,200 children had been evacuated Monday from holiday camps.
Another fire begun in the seaside town of Kouvaras south of Athens forced the evacuation of homes, an equestrian centre and a monastery.
"The extreme weather ... is having a major impact on human health, ecosystems, economies, agriculture, energy and water supplies," said World Meteorological Organization (WMO) Secretary-General Petteri Taalas.
"This underlines the increasing urgency of cutting greenhouse gas emissions as quickly and as deeply as possible."
Too hot at beach
Health authorities in Italy issued red alerts for 20 cities, from Naples in the south to Venice in the north, up from 17 Monday.
At Lanusei, near Sardinia's eastern coast, a children's summer camp was restricting beach visits to the early morning, teacher Morgana Cucca told AFP.
"These days, of course, they're not doing sports, we take them three days to the sea and three days into the woods," she said.
In the Sardinian capital of Cagliari, pharmacist Teresa Angioni said patients were arriving in the morning before the worst heat of the day.
"They mainly buy magnesium and potassium supplements and ask us to measure their blood pressure, which is often low," Angioni said.
Several regions of Spain were placed on red alert by the authorities, with temperatures set to reach a high of 44C in Catalonia and the Balearic Islands. Monday's peak temperature was 44.9C in Andalusia.
In the Canary Islands, firefighters battled to control a wildfire that has ravaged 3,500 hectares of forest.
In parts of Asia, record temperatures have triggered torrential rain.
Nearly 260,000 people were evacuated in southern China and Vietnam before a typhoon made landfall late Monday, bringing fierce winds and rain, but weakening to a tropical storm by Tuesday.
China reported a new mid-July high of 52.2C in the northwestern Xinjiang region's village of Sanbao, breaking the previous high of 50.6C set six years ago.
In Japan, heatstroke alerts were issued in 32 of 47 prefectures, mainly in central and southwestern regions, with at least 60 people were treated for heatstroke, media reported.
The record-setting heat came as US climate envoy John Kerry met with Chinese officials in Beijing, as the world's two largest polluters revive stalled diplomacy on reducing planet-warming emissions.
Speaking at Beijing's Great Hall of the People with China's top diplomat Wang Yi on Tuesday, Kerry called for "global leadership" on climate issues.
'Oppressive' US heat
In the normally hot and dry western and southern US states, more than 80 million people were under advisories for a "widespread and oppressive" heatwave that sent temperatures at California's Death Valley to a near-record 52C Sunday.
In Arizona, state capital Phoenix tied its record of 18 consecutive days above 43C, as temperatures hit 45C Monday.
The US National Weather Service predicts similar highs at least through next weekend, while warning of overnight lows remaining dangerously elevated, above 32C.
In Southern California, several wildfires have ignited over the past few days in rural areas east of Los Angeles.
The biggest, named the Rabbit Fire, had burned nearly 8,200 acres and was 45 percent contained on Monday night, according to authorities.