Tokyo saw three successive days of temperatures topping 35C, marking the worst streak of hot weather in June in around 150 years.
Japan swelters under scorching temperatures for a fourth successive day, as the capital's heat has broken nearly 150-year-old records for June and authorities have warned power supply remains tight enough to raise the spectre of cuts.
A high of 36C was predicted for Tokyo on Tuesday, after three successive days of temperatures topping 35C - the worst streak of hot weather in June since records began in 1875.
Cases of hospitalisation from heatstroke rose early in the day, with many in the capital continue to flout government advice by continuing to wear face masks outdoors - a legacy of more than two years of the Covid-19 pandemic.
For a second day, authorities asked consumers in the Tokyo area to conserve electricity to avoid a looming power cut.
The heatwave comes less than two weeks before a national election in which surging prices - including electricity - are rated the top concern for voters in opinion polls that show the government's approval rating slipping.
As of 0000 GMT (9:00AM local time), 13 people had been taken to hospital with suspected heatstroke, Fuji News Network said.
At least two people are believed to have died from heatstroke, media said, prompting authorities to moderate their calls for power saving.
"Apparently there are some elderly people who have turned off their air conditioners because we are asking people to save energy, but please - it's this hot - don't hesitate about cooling off," trade and industry minister Koichi Hagiuda told a news conference.
Historic and dangerous heatwave gripping Northeast Asia breaks national June record in Japan 🇯🇵— Scott Duncan (@ScottDuncanWX) June 25, 2022
The first 40°C ever to be recorded in the country in the month of June. pic.twitter.com/E4KnKkSLYj
Power supply creaks under strain
The reserve ratio for Tokyo during the evening (1630-1700) on Tuesday was expected to fall below 5 percent as of Monday evening, close to the minimum of 3 percent that ensures stable supply, in Tokyo and eight surrounding prefectures. Reserve capacity below 3 percent risks power shortages and blackouts.
On Tuesday, the Ministry of Economy, Trade and Industry (METI) said predictions had improved slightly, but still called for consumers to be economical with power use.
Monday's warning prompted government offices, including METI, to turn off some lights in the afternoon and evening, with METI haling use of 25 percent of elevators in its building.
Electronics stores took similar steps, shutting off televisions and other goods on sales floors that would normally be kept on to lure buyers, and some Tokyo residents said on social media they were turning off all appliances not in use. But politicians began to call for further steps.
Tokyo governor Yuriko Koike attended a meeting of Tokyo Electric Power Co (TEPCO) shareholders on Tuesday, later saying she had called for price cuts, Fuji News Network reported. TEPCO provides power to the greater Tokyo region.