Japan's women's softball team got the Tokyo 2020 Olympics off to a winning start for the hosts on Wednesday, a boost for the pandemic-postponed Games that the World Health Organization sees as "a celebration of hope" even as COVID-19 cases surge.

Olympics and Japanese officials have forged ahead with the sports spectacle despite opposition in the country to hosting more than 11,000 athletes, staff and media.

But spectators have been barred and restrictions imposed in host city Tokyo in an effort to minimise health risks and contain a mounting number of infections.

WHO head Tedros Adhanom Ghebreyesus said the Games should go ahead to demonstrate to the world what can be achieved with the right plan and measures.

"May the rays of hope from this land illuminate a new dawn for a healthy, safer and fairer world," he said, holding aloft an Olympic Games torch as he addressed the International Olympic Committee members at their session in the Japanese capital. "It is my sincere hope the Tokyo Games succeed."

But Tedros warned the world was in the early stages of another wave of infections and criticised the vaccine discrepancies between countries.

"The pandemic will end when the world chooses to end it. It is in our hands," he said. "We have all the tools we need. We can prevent this disease, we can test for it and we can treat it."

Japan, with about 34% of the population having had at least one dose of the vaccine, has been concerned the Olympics could become a super-spreader event.

In a recent poll in the Asahi newspaper, 68% of respondents expressed doubt about the ability of Olympic organisers to control coronavirus infections, with 55% saying they were opposed to the Games going ahead.

Japanese domestic media reported that government adviser Shigeru Omi said Tokyo daily COVID-19 infections may spike to a record level of 3,000 in the first week of August.

That would cause an extremely high risk of pressuring on the already-stretched medical system.

Underscoring the downsized Games due to the pandemic, Prime Minister Yoshihide Suga will only meet with less than 20 country leaders on the sidelines of the Olympics, down from as many as 120 originally scheduled, the Mainichi Shimbun daily reported.

The softball match between a gold-medal contender Japan and Australia was held amid buzzing cicadas and polite applause from a few hundred staff as a trio of two-run homers cleared the fence after spectators were banned at the stadium.

The game ended after five innings due to a mercy rule.

Players standing along the benches under the scorching sun - 30 degrees Celsius (86 Fahrenheit) by midgame - shouted at the hitters all morning, giving the game a Little League feel.

The match was held in Fukushima, the area devastated by the worst nuclear crisis since Chernobyl, which was triggered by the 2011 earthquake and tsunami.

Two more softball games as well as the first six women's soccer matches are scheduled for later Wednesday. The opening ceremony is Friday.