A blistering summer heatwave in Japan has claimed four lives and seen 900 people hospitalised this week, media said Friday, amid an energy saving campaign due to the Fukushima nuclear disaster.

The mercury has risen above 35 degrees centigrade (95 degrees Fahrenheit) for three days in a row in much of Japan, where the thermostats of most air-conditioners have been turned down to reduce electricity consumption.

More than two-thirds of Japan's nuclear reactors are offline five months after the March 11 quake and tsunami sparked the world's worst nuclear accident since Chernobyl 25 years ago at the Fukushima Daiichi plant.

Four people died of heat stroke on Thursday -- two farmers, a security guard and a construction worker -- said the Asahi daily.

More than 900 people have been taken to hospital with heatstroke symptoms, including about 20 in serious condition, said the Kyodo News agency.

Since the start of July, more than 21,000 people in Japan have been rushed to hospital due to the summertime heat, and 32 of them have died, according to data from the Fire and Disaster Management Agency.

This weekend is one of the busiest travel times of the year in Japan, when millions return to their home towns for the Obon festival to honour their ancestors, and when many take their summer holiday.