STORY: "The discharge of radioactive contaminated water is not just a problem for Japan, and of course not just for Fukushima, it's a global problem, so I think it's necessary for everyone to think about it," said Fujita, a 61-year-old doctor from Fukushima, who joined the protest.
The water was mainly used to cool reactors in the aftermath of the disaster.
It is being stored in huge tanks at the plant and is enough to fill about 500 Olympic-sized swimming pools.
Officials say the tanks are near capacity and have to be removed for reconstruction.
The water is treated, filtered and diluted and the plant's owner and the Japanese government say it is safe, although it does contain traces of tritium.
That has done little to ease the concerns of opponents to the plan who held signs with slogans such as "Don't release contaminated water into the ocean!"