All lawmakers of the main opposition Democratic Party were set to launch an overnight sit-in at the National Assembly on Thursday to protest Japan's plan to discharge radioactive water from the crippled Fukushima nuclear power plant into the sea.

The protest comes after the International Atomic Energy Agency said in a report earlier this week that Japan's plan to treat contaminated water and release it into the ocean meets international safety standards and its impact on people and the environment would be negligible.

The report was seen as a blessing for the water discharge expected to begin sometime this summer.

During the overnight protest set to begin at 7 p.m. and last until Friday at noon, 167 DP lawmakers plan to hold a "relay filibuster," in which lawmakers take turns to deliver long speeches demanding the withdrawal of Japan's plan and denouncing Seoul's response to it.

The DP also plans to hold a protest rally outside the National Assembly building on Saturday, bringing together not only lawmakers, but also aides, staffers and regional party officials.

Separately, some members of Japan's minor opposition Social Democratic Party were to arrive in Seoul on Thursday to support the campaign against the release. They plan to visit DP lawmakers during the sit-in and hold a round table discussion.

The government of President Yoon Suk Yeol has said it respects the findings in the IAEA report.

Officials have stressed that as long as the treatment and release plan goes ahead as planned, it will not have any harmful effects on S. Korea because the released water would be massively diluted in ocean waters and sea currents would carry it away before reaching S. Korean shores years later.

The ruling People Power Party has accused the DP of inciting fears of bad health consequences among the people with unscientific claims in an attempt to take advantage of the issue for political gains ahead of next year's general elections.

In contrast, the DP has argued the release will harm the environment and health of people.

DP floor leader Rep. Park Kwang-on urged the government on Thursday to bring up the issue at the upcoming Consultative Meeting of Contracting Parties to the London Convention and Protocol, an agreement aimed to protect the marine environment from human activities such as dumping. (Yonhap)