Intense heat gripped much of Japan on Saturday, with the mercury topping 37 degrees Celsius in the western parts of the country.
High temperatures that require preventive measures against heatstroke are expected in eastern Japan on Sunday.
The Meteorological Agency says a high-pressure system in the Pacific Ocean is covering the western and eastern parts of the country, raising temperatures.
The daytime high reached 37.8 degrees in Nishitosa in Shimanto City in Kochi Prefecture, 37.6 degrees in Mima City in Tokushima Prefecture and Motoyama Town in Kochi Prefecture, and 35.5 degrees in Koshigaya City in Saitama Prefecture and Tokyo's Fuchu City.
Temperatures are expected to remain above 25 degrees all night both in western and eastern Japan.
Weather officials are calling on people to use air conditioners even at night, and to rehydrate frequently.
Temperatures are expected to rise further in some parts on Sunday.
The daytime high is expected to reach 39 degrees in Chichibu City in Saitama Prefecture, and 38 degrees in Nagoya City, Shizuoka City, Kofu City, Kumagaya City in Saitama Prefecture and Maebashi City.
The Meteorological Agency and the Environment Ministry issued heatstroke alerts for 20 prefectures, including Tokyo, Shizuoka, Aichi, Ehime and Miyazaki.
People are advised to refrain from intense exercise and unnecessary outings.
Officials also say extremely unstable atmospheric conditions caused by high daytime temperatures coupled with inflows of warm, damp air are expected in wide areas from western to northern Japan through Saturday night.
They are advising people to be on the alert for lightning strikes, wind gusts, including tornadoes, as well as heavy rain and hail.