South Koreans rallied near the Japanese embassy in the capital Seoul to protest against Japan's plan to discharge wastewater from the crippled Fukushima power plant, a day before the scheduled release into the Pacific Ocean.

-Final preparations underway for Fukushima water release-

Final preparations to discharge wastewater from the crippled Fukushima power plant in Japan were underway Wednesday, its operator said, a day before the scheduled release into the Pacific Ocean.

Tokyo had announced Tuesday that the operation would begin on Thursday, prompting an angry response from China and partial import bans on Japanese seafood by Hong Kong and Macau.

The operator of the plant, TEPCO, said that it diluted late Tuesday a cubic meter of the wastewater with around 1,200 cubic meters of seawater and allowed it to flow into position in a pipe.

This water will be tested and then from Thursday released into the Pacific Ocean together with more water stored at the site that will be transferred and diluted, TEPCO said in a statement.

Media reports said the operation would begin around 1:00 pm, while the operator declined to confirm the reports.

The Fukushima-Daiichi nuclear power station was knocked out by a massive earthquake and tsunami that killed around 18,000 people in March 2011, sending three of its reactors into meltdown.

TEPCO has since collected 1.34 million cubic meters of water -- almost 540 Olympic swimming pools' worth -- used to cool what remains of the still highly radioactive reactors, mixed with groundwater and rain.

A special system has filtered out all radioactive nuclides except for tritium, levels of which will be well below that released by nuclear power plants in their normal operations -- including in China, TEPCO says.

The release has been endorsed by the UN's nuclear watchdog -- the International Atomic Energy Agency (IAEA) -- which said it will have staff on-site on Thursday.

With around 1,000 steel containers holding the water at the site, TEPCO says that it needs to clear space for the next stage of the lengthy, expensive and risky decommissioning of the plant.

That is the removal of molten nuclear fuel and radioactive rubble from the wrecked reactors, an operation so dangerous that TEPCO will need to use robots and not humans.

The plant operator will carry out four releases of treated water from Thursday until March 2024, with 7,800 cubic meters of water released each time, its documents showed.

The first discharge will take about 17 days. About five trillion becquerels of tritium will be released this fiscal year, TEPCO added.