Among the protesters was Sachiko Uno, who told Xinhua that she came to "stop them from polluting the ocean."

"One of the world's top-level nuclear accidents occurred in Fukushima despite safety promises made by the government and TEPCO. Now, they have become the world's first by beginning to discharge nuclear-contaminated water into the ocean. This is a 'double damage,'" said the 73-year-old woman.

Uno is also concerned with TEPCO's credibility. "It says the radioactivity concentration in the nuclear-contaminated water meets the standards, but I can't believe that at all. The reliability of the measuring instruments themselves is questionable," she said.

Hit by a magnitude-9.0 earthquake and an ensuing tsunami on March 11, 2011, the Fukushima plant suffered core meltdowns that released radiation, resulting in a level-7 nuclear accident, the highest on the International Nuclear and Radiological Event Scale.