Hundreds of South Korean activists gathered in central Seoul on Saturday to protest against Japan's plan to release treated radioactive water from the tsunami-wrecked Fukushima nuclear power plant into the ocean.

Japan's Asahi Shimbun daily reported earlier this week that the country plans to start releasing the water into the ocean as early as late August, citing unidentified government sources.

"If it is discarded, radioactive substances contained in the contaminated water will eventually destroy the marine ecosystem," said Choi Kyoungsook of Korea Radiation Watch, an activist group that organised the protest.

"We are opposed... because we believe the sea is not just for the Japanese government, but for all of us and for mankind."

Hundreds of protesters held up signs saying "Keep It Inland" and "Protect the Pacific Ocean!" while singing songs and listening to rally organisers.

Japan's nuclear regulator last month granted approval for plant operator Tokyo Electric Power to start releasing the water, which Japan and the International Atomic Energy Agency said is safe but nearby countries fear may contaminate food.

U.S. President Joe Biden will meet Japanese Prime Minister Fumio Kishida and South Korean President Yoon Suk Yeol for a trilateral summit on Aug. 18.

"There is talk that the dumping of contaminated water is on the summit agenda. The governments of South Korea, the U.S., and Japan should view it an environmental disaster, rather than a political issue, and agree to block it... for future generations," Choi said.