Originally Published: 20 JUL 22 07:38 ET

Updated: 20 JUL 22 13:24 ET

By Rob Picheta, CNN

(CNN) -- Hundreds of millions of people around the world were sweltering in extreme heat on Wednesday, as record-breaking heat waves set swathes of Europe's countryside on fire, scorched the US and put dozens of Chinese cities under alert.

Five separate high-pressure weather systems across the northern hemisphere, which are linked by atmospheric waves, have led to unprecedented temperatures on multiple continents. The UK smashed its all-time heat mark on Tuesday, as did several cities in the Texas and Oklahoma, including Wichita Falls, which reached a broiling 115 degrees Fahrenheit (46.1 degrees Celsius) on Tuesday.

As Europe's heat wave moves eastwards, wildfires have forced tens of thousands of people from their homes, blanketing parts of Italy, Greece and France in smoke. Germany, Hungary and Italy have been hit with high temperatures of around 100 F (nearly 38C) in places.

The European Forest Fire Information System put 19 European countries on "extreme danger" alerts for wildfires on Wednesday, across an expanse stretching from Portugal and Spain in the southwest to Albania and Turkey in the southeast.

There was some respite in the UK, where temperatures dipped from an all-time high of 40.3 C (104.5 F) on Tuesday back into the 20s. But some residential areas around London were left in ruins after fires broke out across parts of the capital, stretching the fire service to its limits.

"Yesterday was the busiest day for the fire service in London since the Second World War," London's mayor Sadiq Khan told Sky News on Wednesday, as residents of the capital watched their homes destroyed in heat-triggered blazes they never thought possible.

A resident of Wennington, a London suburb affected by Tuesday's fires, told CNN that the gardens on his street were "like a tinder box" in the days leading up to the fire. Stock lost his home, eight chickens and two beehives when the fire broke out.

"I didn't sleep last night. I was in the hotel room thinking how bad it could have gone. I just thank god that everyone got out alive," he said. "We've lost everything. But when we get back, we can clear the site, put some fences up, get a couple of mobile homes and we'll start again."

London had no available fire engines at one point in the afternoon amid unprecedented demand, a senior fireman with the London Fire Brigade's special rescue team told CNN.

In the United States, local leaders are urging caution and issuing health warnings as a heat wave that shows no sign of slowing before the weekend continues to bake the south-central regions of the country.

And in China, millions of densely-populated cities are responding to extreme heat. According to the China Meteorological Administration, at least 31 Chinese cities issued the second-highest orange alert warning, with temperatures expected to go up to 37 degrees Celsius (98 degrees Fahrenheit) in the next 24 hours.

Europe burns in record heat

Greece: On the outskirts of the Greek capital, Athens, firefighters have been tackling vast blazes that are being whipped up by wind. At least 600 people have been evacuated, including from a children's hospital, authorities said. One person has died and 30 have been transferred to hospitals in the capital's Attica region, the Greek Fire Service said on Wednesday.

"Our top priority remains the safeguarding of human lives. But also that of vital public infrastructure as well as citizens' properties," spokesperson Ioannis Artopoios said during an earlier televised briefing.

Huge clouds of smoke remain visible in the city on Wednesday, despite the efforts of hundreds of firefighters. Romanian fire crews have been drafted in to assist the operation.

Italy: Blazes are also being tackled in parts of Italy. Wildfires in Tuscany caused gas tanks to explode and forced evacuations overnight, according to the regional President Eugenio Giani.

Germany: In Alsdorf, western Germany, three residents and two firefighters were injured in a blaze on Tuesday, and much of the country is primed for more fires as temperatures rise on Wednesday.

France: In France, aircraft continued to dump water over burning landscapes. Fires have been raging there for a week now, though they advanced "very little" on Tuesday night in the Gironde region, according to local authorities. Smoke swirled over the Brennilis nuclear power plant in Brittany on Wednesday morning.

Just as UK Prime Minister was criticized for a lack of preparedness for the heat wave, France's Emmanuel Macron too is coming under pressure to respond more quickly to the heat and fires, which have already burned 25 times more land in France than in the same period last year, government spokesperson Olivier Veran told journalists on Wednesday.

On Tuesday, the president of the fire-stricken Gironde region called for additional resources, including firefighting aircraft to be diverted there.

A campsite owner told CNN affiliate BFMTV on Tuesday that "we have demands" for Macron, who will be making a visit to the ravaged southwest on Wednesday.

"We hope that he will be able to very quickly order the public services to help us, to support us in the administrative steps to put the camp site in working order as soon as possible," said Stephane Carella, co-owner of Pyla Camping whose site was destroyed by fires.

"Everything has gone up in smoke," he said, with some 90% of his property affected by the fire. Carella described the remainders of the site as "apocalyptic."

High temperatures rip through US

The baking weather in the US has so far been centered on parts of the south, and is raising particular concern about the welfare of elderly, vulnerable and homeless people.

In Texas, 14 prisons have no air conditioning and 55 have only partially-working systems, the Texas Department of Criminal Justice (TDCJ) told CNN in an email. Texas has had at least four heat waves this season, a hot streak that started impacting the state before the official start of summer. Since May 1, more than half their days have come with some level of heat alert.

As temperatures in neighboring Arizona reached triple digits on Tuesday afternoon, around 7,000 people lost power due to strong storms, a spokesperson for the Arizona Public Service Company said.

In some areas, such as Miami-Dade County and Phoenix, local governments have hired chief heat officers to help residents combat the heat.

Much of the country's north, and parts of Canada, are preparing for temperatures to soar too. Philadelphia declared a "heat caution" starting noon Tuesday and extending to until 8 p.m. ET Thursday, the city said in an email to CNN. It also declared a "code red" alert for homeless people in the area.

The Canadian government issued heat and severe thunderstorm advisories in at least four provinces on Tuesday.