The main opposition Democratic Party on Friday raised intensive questions about the credibility of a recent International Atomic Energy Agency (IAEA) report on Japan's radioactive water release, as the IAEA chief is set to begin his three-day visit to South Korea later that day.

Ranking DP officials poured out harsh criticism against the IAEA report during the party's supreme council meeting held after almost all of its lawmakers staged an overnight sit-in at the National Assembly to protest Japan's plan to discharge radioactive water from the crippled Fukushima nuclear plant into the ocean.

They also urged the Yoon Suk Yeol government to declare opposition to the neighboring country's planned water release.

"Should the release of contaminated Fukushima water be permitted on the basis of an irresponsible report?" DP leader Lee Jae-myung asked, demanding that the Yoon government oppose the discharge for the sake of national interest.

DP floor leader Rep. Park Kwang-on asked IAEA Director General Rafael Mariano Grossi to answer who will be responsible for the outcome of the water discharge.

DP Rep. Wi Seong-gon, head of a party committee on Japan's water discharge plan, said he plans to meet with Grossi on Sunday. Grossi is set to arrive in Seoul later Friday for a three-day visit following a trip to Japan.

"I will deliver our citizens' concerns over the planned release of contaminated water," he told Yonhap News Agency over the phone.

Rep. Jung Chung-rai, a member of the DP supreme council, warned those who trust the IAEA report may pay dearly later, saying, "The IAEA is not a hospital. If a disease occurs from the contaminated Fukushima water, the IAEA will not treat it or take responsibility."

Following the supreme council meeting, DP lawmakers and officials gathered in front of the National Assembly to hold a rally against Japan's discharge of radioactive water. The party put the number of people attending the rally at about 1,500.

Grossi's visit comes on the heels of the agency's conclusion that Japan's plan to release treated radioactive water into the sea is consistent with international safety standards. The Seoul government also said Friday that the Japanese plan would meet international standards, including those set by the IAEA, if carried out as planned. (Yonhap)