SEOUL -- Thousands of ordinary South Korean people, activists and fishermen gathered Saturday in central Seoul from across the country to express their firm opposition to Japan's planned discharge of radioactive wastewater into the ocean.

Participants in the protest rally shouted slogans in unison and held placards that read "Desperately oppose dumping radioactive wastewater into the ocean," and "Keep it on land, not marine dumping."

"Nobody knows how far (the wastewater) will go after it is discharged into the ocean. It will destroy all ecosystems, so it will be a disaster for all humankind," Kim Young-ran, a rally participant in her 50s, told Xinhua.

Kim, who was working in social welfare for fishermen in the southwestern port city of Mokpo, noted that some fishermen already stopped fishing in the city due to growing concerns that discharge of the nuclear-contaminated wastewater is imminent.

Despite widespread criticism from both home and abroad, the Japanese government has been pushing to dump the nuclear-contaminated wastewater this summer from the crippled Fukushima Daiichi nuclear power plant, which was hit by a massive earthquake and an ensuing tsunami in March 2011.

Kim Min-kyung in her 20s, the chief of a South Korean university expedition team to oppose Fukushima radioactive wastewater discharge, told Xinhua that a lot of Japanese university students and citizens, whom she met in Japan as head of the expedition team, took the wastewater dumping issue very seriously.

"The ocean is not Japan's but belongs to all of us. But, (Japan) would discharge it without any approval or agreement from neighboring countries. I think it goes too far to do it with nobody knowing how it will affect our country and other neighboring countries," she said.