From North America to Europe and Asia, people gulped water and sought shelter from the sweltering heat, as temperatures crept toward record highs.
Europe, the globe's fastest-warming continent, was bracing for its hottest-ever temperature this week on Italy's islands of Sicily and Sardinia, where a high of 48 degrees Celsius (118 degrees Fahrenheit) is predicted, according to the European Space Agency.
Predictions of historic highs in the coming days led the health ministry to sound a red alert for 16 cities including Rome, Bologna and Florence.
"We're from Texas, and it’s really hot there, we thought we would escape the heat but it’s even hotter here," Colman Peavy, 30, said as he sipped a cappuccino at an outside terrace in central Rome with his wife Ana at the start of a two-week Italian vacation.
The mercury was due to hit 40C after noon on Monday in Rome, where some 15,000 braved the temperatures the previous day to hear Pope Francis lead prayers, using parasols and fans to keep cool.
A new heatwave is expected in Greece from Thursday and meteorologists warned of a heightened risk of wildfires amid strengthening winds from the Aegean Sea.
In Romania, temperatures are expected to reach 39C on Monday across most of the country.
Little reprieve is forecast for Spain, where meteorologists warned that "abnormally high" temperatures on Monday, including up to 44C in the southern Andalusia region in what would be a new regional record.
In Japan, heatstroke alerts were issued in 32 out of the country’s 47 prefectures, mainly in central and southwestern regions, as scorching temperatures continued Monday.
At least 60 people in Japan were treated for heatstroke, local media reported. Toyota City, home to the leading automaker, had recorded the country’s highest temperature for the day of 39.1 degrees Celsius, as television broadcasters urged people to stay indoors and avoid the life-threatening heat.
Japan's highest-ever temperature was 41.1C first recorded in Kumagaya city in 2018.
In western and southern states in the United States, which are used to high temperatures, more than 80 million people were under advisories as a "widespread and oppressive" heatwave roasted the region.
California's Death Valley, often among the hottest places on Earth, reached a near-record 52C Sunday afternoon.
In Arizona, the state capital Phoenix recorded its 17th straight day above 109 degrees Fahrenheit (43 degrees Celsius), as temperatures hit 113F (45C) on Sunday afternoon.
Southern California was fighting numerous wildfires, including one in Riverside County that has burned more than 7,500 acres (3,000 hectares) and prompted evacuation orders.
Along with the heat, parts of Asia have also been battered by torrential rain.
South Korea's president vowed Monday to "completely overhaul" the country's approach to extreme weather, after at least 40 people were killed in recent flooding and landslides during monsoon rains, which are forecast to continue through Wednesday.
In northern India, relentless monsoon rains have reportedly killed at least 90 people, following burning heat.
Major flooding and landslides are common during India's monsoons, but experts say climate change is increasing their frequency and severity.
China on Sunday issued several temperature alerts, warning of 40-45C in the partly desert region of Xinjiang, and 39C in southern Guangxi region.
It can be difficult to attribute a particular weather event to climate change, but many scientists insist that global warming is behind the intensification of heatwaves.
The EU's climate monitoring service said the world saw its hottest June on record last month.