Japan’s blistering heatwave has claimed its 80th life, according to authorities in the country where thousands have also been rushed to hospital because of the high temperatures.

Officials have urged citizens to stay indoors to avoid temperatures, which have exceeded 40C in some areas.

The government said it might pay to help state schools install air conditioners and suggested extending the summer holiday, which started this week for many students.

It comes as the Asian nation’s weather agency declared the heatwave, a natural disaster.

“Record temperatures are continuing across the country and emergency measures to protect students and their well-being has become an issue,” chief cabinet secretary, Yoshihide Suga, told a news briefing.

Public broadcaster NHK has also advised frequent sips of water to keep hydrated and said people should restore salt levels depleted by sweat and stay indoors.

It has also aired videos with instructions on treating victims of mild heat-stroke.

Temperatures came close to 40C earlier this week in many cities, just short of Monday’s record of 41.1C in the city of Kumagaya, northwest of the capital, Tokyo.

A kimono-clad woman using a sun umbrella walks on a street during a heatwave in Tokyo (Issei Kato/Reuters)

In central Tokyo, where an Olympic stadium is being built for the 2020 games, temperatures hovered near 35C.

People turned up their air conditioning, driving prices on the Japan Electric Power Exchange to their highest in five years, exchange data showed, the second week that they have hit multi-year highs.

More people are dying from the heat, figures from the Fire and Defence Management Agency (FDMA) show. In the week to July 22, 65 people died, versus 12 the previous week and only three in the seven days prior, the FDMA said.

A man wipes the sweat from his face in the scorching heat at a business district in Tokyo (AP)

At least 13 more people died on Monday, the Kyodo newswire reported.

The heat is expected to ease slightly this week as a high pressure zone responsible for it moves west and east, allowing moist air to bring rain to some scorched areas, the Japan Meteorological Agency said.