TOKYO — The hibakusha, as the survivors of the atomic bombings of Hiroshima and Nagasaki are known in Japan, have achieved a powerful feat of alchemy, transforming their nightmarish memories of the blasts and their aftermath into a visceral force for promoting a world free of nuclear arms.

Each year for over half a century, many of them have gathered in the early hours of Aug. 6 at the Hiroshima Peace Memorial Park to mourn the city’s destruction by the American military during World War II, and to serve as a living testament to the abiding dangers of the bomb.

But on Thursday, as Hiroshima marks the 75th anniversary of the nuclear assault, the hibakusha will be a diminished presence, a victim of the twin forces of the coronavirus pandemic and advancing age.

“There were people who questioned whether it was OK for hibakusha to participate in the ceremony in the midst of the pandemic,” said Kunihiko Sakuma, chair of the Hiroshima branch of the Japan Confederation of A- and H-Bomb Sufferers’ Organizations.

Despite the health risks, a relatively small number of survivors will attend this year, Mr. Sakuma said. They believe that “they’ve come this far” and “can’t quit,” he said, adding that “sending this message from Hiroshima is extremely important.”

City officials and peace activists had envisioned a series of grand events to commemorate what will most likely be the last major anniversary of the bombing for almost all of the hibakusha (pronounced hee-bak-sha) still living.

But the coronavirus has forced them to curtail the events, moving conferences on nuclear disarmament online, canceling or postponing related meetings and reducing the number of attendees to around 800, one-tenth of the turnout during a normal year.

Cognizant of the declining population of survivors of the two atomic bombings, which now stands at about 136,000, the Hiroshima government decided to focus this year’s remembrance on mourning the dead and honoring the experience of those who remain.

The memories of the hibakusha, who now average 83 in age, are an increasingly precious resource. As their numbers fall, they and their supporters are being forced to envision what the disarmament movement will look like without the people who have put a human face on the cost of nuclear war.

Mr. Sakuma said he hoped that the survivors’ children and their children’s children would carry on the fight as long as it took.

“The hibakusha can’t avoid the fact that our numbers are decreasing,” he said. “Each year a few thousand more disappear. Who knows how many years we have left?”

Scarred physically and mentally by the tremendous power unleashed by the splitting of atoms over Hiroshima and Nagasaki, the hibakusha have become a rallying point for peace activists the world over, as well as the moral ballast of Japan’s postwar pacifism.

Survivors have spent measureless time and energy campaigning for the complete elimination of nuclear weapons. From welcoming visitors into their homes in Hiroshima and Nagasaki to lecturing on cruise ships, they have shared their message of peace with audiences at home and abroad, including with the world’s political and religious leaders.

For both policymakers and the public, hearing survivors’ firsthand experiences of bombings that killed more than 200,000 people has been “really important on a personal level,” said Sharon Squassoni, director of the global security program at the Union of Concerned Scientists. “It’s really easy for these issues to become abstract because these weapons haven’t been used in 75 years.”

When survivors’ organizations first began to be politically active in the 1950s, they had two goals: to demand compensation and financial support from the Japanese government, and to push for the elimination of nuclear weapons.

They have been largely successful on the first front, although some compensation claims are still wending their way through the country’s courts.

But after years of optimism fed by signs of progress, most survivors now say that a world free of nuclear weapons is a distant dream. That bleak outlook reflects a general feeling in the arms-control community that the world is giving up hard-won gains.

The number of nuclear warheads has dropped from a peak of around 70,000 in the mid-1980s to about 13,000 today. But in the past 25 years, India, Pakistan and North Korea have established themselves as nuclear states, China has expanded its modest arsenal and, most important, the United States and Russia — far and away the largest nuclear powers — have begun extricating themselves from treaties that have bound them since the end of the Cold War.

Those trends, however, have only steeled the survivors’ resolve to fight. In 2017, their efforts were rewarded with passage in the United Nations General Assembly of the Treaty on the Prohibition of Nuclear Weapons.

The treaty’s future is uncertain. It has only 40 of the 50 signatures it needs before it can come into effect. And it is unlikely to ever gain support from the nuclear-armed states or from countries, like Japan itself, that are under the aegis of the American arsenal.

For the hibakusha, though, the treaty is a validation. The survivors had long believed that “no one was listening to them,” said Kazumi Mizumoto, an expert on security studies and nuclear disarmament at Hiroshima City University. But the treaty’s passage “reaffirmed their existence,” he said.

Still, that existence is facing the inevitable toll of time. As the ranks of hibakusha shrink, their lobbying groups have begun to fall on hard times. One disbanded in June 2019, citing the difficulties of continuing with an aging leadership.

“We’re coming to the point where we have to think about how our organizations can continue forward. The situation is tough,” said Koichiro Maeda, 71, a former director of the Hiroshima Peace Memorial Museum and the current head of the secretariat of one of the survivors’ groups.

It is more important than ever to ensure that the survivors’ legacy is carried on, said Maika Nakao, a professor of history at Nagasaki University who studies Japan’s relationship with nuclear weapons.

In addition to their role on the international stage, the survivors, and their stories, are an integral part of Japan’s national identity, serving as the country’s conscience in an era when the reasons for adhering to principles of peace have become more and more abstract.

“We have to think about how to acknowledge the history, how to memorialize it and how to pass it down to the future generations,” Professor Nakao said.

“We have a lot of testimonies, but it’s not enough. There is no perfect condition. No matter how much you ask, no matter how much you collect, it’s never enough. It’s important to document everything,” she said.