TOKYO (REUTERS) - A worker at a Tokyo Olympics construction site died on Thursday after being found unconscious while working outside, organisers said on Friday (Aug 9), with media saying heatstroke was suspected as the Japanese capital swelters through a deadly heatwave.

Soaring temperatures have killed at least 57 people across Japan since late July, just a year before Tokyo hosts the 2020 Games, highlighting the possible health threat to athletes and fans that is one of the biggest challenges for Olympics planners.

A 50-year-old construction worker who'd been laying cable outside Tokyo Big Sight, which is being renovated to serve as the media centre for Tokyo 2020, was found unconscious on the ground on Thursday afternoon and taken to hospital, where his death was confirmed, Tokyo 2020 organisers said in a statement.

"The cause of death is still unknown," spokesman Masa Takaya said in an email, adding that they would provide more details once the cause of death was clear.

Police quoted by national broadcaster NHK said conditions at the site suggested heatstroke may have been the cause, but police declined to confirm the report.

Temperatures in Tokyo hit 35.5 degrees Celsius on Thursday.

Since July 24, temperatures in the metropolitan city have clung above 31 deg C, exactly a year before the Games are set to open, with the heat intensifying in August to average daily highs of 34.8 deg C.

At least 57 people died across Japan from the heat between July 29 to Aug 4 and 18,347 were taken to hospital. Some 45 people have died in Tokyo alone since Aug 1, NHK said, many of them elderly living on their own.

Most of the Summer Olympics have been held during the northern hemisphere summer, but when Tokyo hosted the Games previously, in 1964, they were shifted to October to avoid the summer heat. The Mexico 1968 Olympics were also held later due to weather conditions.

Such shifts are no longer possible, though, due to international broadcasting and sports schedules, including the US American football season and European soccer. Athens 2004 and Beijing 2008 were held in summer despite the climate of both cities.

Olympics organisers plan to take steps to deal with the heat, including shaded rest areas, tents at security checkpoints, mist sprays and ice packs.

The start times for the men's and women's marathons, on Aug 9 and 2 respectively, have been moved to 6am as an additional precaution.

Tokyo is hosting the Olympics for the second time, with the Games running from July 24 to Aug 9.