"We are not just talking about something that happened in the past. This issue continues to affect us today," Toshihiro Inoue, an official with the Japan Congress Against A- and H-Bombs, told AFP.
The 23-man tuna boat Lucky Dragon Five was sailing 160 kilometersmiles) east of Bikini Atoll, part of Marshall Islands chain in the mid-Pacific Ocean when the United States tested its biggest-ever hydrogen bomb on March 1,
The bomb, codenamed Bravo, was 1,000 times more powerful than the atomic bomb dropped on Hiroshima in 1945, and the Lucky Dragon's crew were exposed to radioactive falling ash, with radio operator Aikichi Kuboyama dying several months later at the age of 40.
Only 11 of the crew members are still alive, many of them suffering from liver and other diseases that their supporters claim are caused by radioactivity from the bomb experiment. Most of their dead colleagues died in their 40s or 50s from cancer, liver disease or hepatitis.
Inoue said the experience of the ship is still relevant today.
"We are talking about 20,000 nuclear bombs that are known to exist today. We are talking about the US plan to develop mini-nukes (small tactical nuclear arms). We must not forget the threats of nuclear weapons," he said.
"The nuclear weapons threat is real and we must stop its spread," he said.
Some 2,000 Japanese activists held a peace walk Monday in the port city of Yaizu, the Lucky Dragon's home port located roughly 160 miles (100 miles) southwest of Tokyo, to visit Kuboyama's grave.
Former crew member Matashichi Oishi, 70, visited the graves of three crew members and urged the government to recognise them and surviving crew members as victims of radiation exposure, known in Japan as "hibakusha," the Kyodo News agency said.
The surviving victims of Hiroshima and Nagasaki atomic bombs are recognised as "hibakusha" and receive medical and financial support from the government.
In the Marshall Islands, flags flew at half-mast to mark the anniversary.
A police honor guard led nuclear survivors, anti-nuclear activists, a high-level US church delegation, and local students on a march down main street to the capital building for a day-long program of speeches and music.
Bikini itself can now be visited for limited periods of time but its food crops still cannot be eaten as they remain tainted by radioactivity.