Rafael Grossi, the director general of the International Atomic Energy Agency (IAEA), arrived in Seoul on Friday after a stay in Japan during which his agency approved Tokyo's plan to release treated water from the tsunami-hit nuclear plant into the sea over the next few decades.

South Korea conducted its own separate review of Tokyo's plan and also concluded that Japan would meet or exceed key international standards, with the release having "negligible consequences".

Despite that, Grossi's visit encountered resistance in South Korea, where public concern over the planned release has grown.

There has been "no disagreement" among experts involved in the review that gave the green light for a decades-long project, the IAEA chief said in an interview with the Yonhap news agency on Saturday.

"This is the final comprehensive report... No experts have come to me saying he or she disagrees on the contents," he said.

"It was a very thorough process."

That was not enough to keep hundreds of protesters from taking to the streets in central Seoul, bashing the IAEA's review as "insufficient" as Grossi met Foreign Minister Park Jin.

Demonstrators held various signs critical of the IAEA and Japan's plan, one of which read "IAEA is not qualified to verify environmental standards".

The IAEA report "was drawn up under the influence of Japan", claimed one protester on a microphone, without giving details.

Opposition MPs have also waged a public campaign in protest of Tokyo's plan, and some have even been on hunger strike.

Grossi is expected to meet opposition lawmakers on Sunday at the parliament.

Some 1.33 million cubic metres of groundwater, rainwater and water used for cooling has accumulated at the Fukushima site, which is being decommissioned after several reactors went into meltdown following the 2011 tsunami which badly damaged the plant.

The plant operator treats the water to remove almost all radioactive elements except tritium, and plans to dilute it before discharging it into the ocean over several decades.

Since taking power last year, South Korean President Yoon Suk Yeol has sought to bury the historical hatchet with Japan on issues including wartime forced labour, as he seeks closer regional security cooperation in the face of rising nuclear threats from North Korea.