Japan began releasing wastewater from the crippled Fukushima nuclear plant into the Pacific Ocean on Thursday despite angry opposition from China and local fishermen.

The start of the discharge of around 540 Olympic swimming pools' worth of water over several decades is a big step in decommissioning the still highly dangerous site 12 years after one of the world's worst nuclear accidents.

Live video provided by plant operator TEPCO showed two engineers clicking on computer mouses and an official saying -- after a countdown -- that the "valves near the seawater transport pumps are opening".

Monitors from the UN atomic watchdog, which has endorsed the plan, were due to be on site for the procedure, while TEPCO workers were scheduled to take water samples later on Thursday.

Japanese officials have repeatedly insisted the wastewater release is safe.

But China's environment ministry on Thursday blasted Japan as "extremely selfish and irresponsible", saying it would "track and study" the impact of the release on its waters.

Ahead of the operation, about 10 people held a protest near the site and around 100 others gathered outside TEPCO headquarters in Tokyo, AFP journalists said.

"It's like dumping an atomic bomb in the ocean. Japan is the first country that was attacked with an atomic bomb in the world, and the prime minister of the country made this decision," said Kenichi Sato, 68.

- Multiple meltdowns -

With around 1,000 steel containers holding the water, TEPCO has said it needs to clear space for the removal of highly dangerous radioactive nuclear fuel and rubble from the wrecked reactors.

Three of the reactors at the Fukushima-Daiichi facility in northeastern Japan went into meltdown following a massive earthquake and tsunami that killed around 18,000 people in 2011.

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Since then, TEPCO has collected 1.34 million cubic metres of water that was contaminated as it cooled the wrecked reactors, along with groundwater and rain that has seeped in.

TEPCO will carry out four releases of the treated water from Thursday until March 2024. The first will last about 17 days, though it is expected to take around 30 years for all of the wastewater to be discharged.

Japan says that all radioactive elements have been filtered out except the tritium, levels of which are harmless and lower than what is discharged by operational nuclear power plants -- including in China.

This is backed by most experts.

"When released into the Pacific, the tritium is further diluted into a vast body of water and would quickly get to a radioactivity level which is not discernibly different from normal seawater," said Tom Scott from the University of Bristol.

- Sushi safety -

Not everyone is convinced, with environmental group Greenpeace saying that the filtration process is flawed, and China and Russia suggesting the water be vaporised and released into the atmosphere instead.

China has accused Japan of treating the Pacific like a "sewer", and even before the release, Beijing banned food imports from 10 out of 47 Japanese prefectures and imposed radiation checks.

Hong Kong and Macau, both Chinese territories, followed suit this week.

Restaurants in Beijing and Hong Kong serving sushi and sashimi are already reeling from the restrictions.

"About 80 percent of the seafood products we use come from Japan," Hong Kong caterer Jasy Choi, who runs a small kitchen for takeaway Japanese food, told AFP.

Analysts said that while China may have genuine safety concerns, its strong reaction is also motivated at least in part by its economic rivalry and frosty relations with Japan.

The South Korean government, which is seeking to improve ties with Japan, has not objected, although many ordinary people are worried and have staged protests.

Social media posts in China and South Korea have included false claims about the release, including doctored images of deformed fish with claims they were linked to Fukushima.

People in the Japanese fishing industry oppose the release, worrying that governments and consumers will shun their seafood.

"I am worried about the future," protestor Ruiko Muto, 70, told AFP in Miharu near the power plant.

"We can't pass on the responsibility of what happened during our generation to the generation of our children and to future generations."

China halts all Japanese seafood imports over 'selfish' Fukushima release

China on Thursday banned all Japanese seafood imports over what it said was the "selfish" release of wastewater from the crippled Fukushima nuclear plant.

Japan began discharging treated contaminated water from the stricken plant into the Pacific Ocean on Thursday in an operation it insists is safe but which has generated a fierce backlash from China.

Beijing's foreign ministry branded the release as an "extremely selfish and irresponsible act", saying in a statement that the move would "push the risks onto the whole world (and) pass on the pain to future generations of human beings".

And China's customs authority said it would "suspend the import of aquatic products originating in Japan from August 24, 2023, including edible aquatic animals".

The decision was taken to "comprehensively prevent the food safety risks of radioactive contamination caused by the discharge of nuclear wastewater from Fukushima into the sea", the General Administration of Customs said.

It would also "protect the health of Chinese consumers and ensure the safety of imported food", the authority added.

Beijing had already suspended all food imports from 10 out of 47 Japanese prefectures in July, with Hong Kong following suit.

China imported over $500 million worth of seafood from Japan last year, according to customs data.

In 2011, three reactors at the Fukushima-Daiichi facility in northeastern Japan went into meltdown following a massive earthquake and tsunami that killed around 18,000 people.

Since then, plant operator TEPCO has collected 1.34 million cubic metres of water contaminated as it cooled the wrecked reactors, along with groundwater and rain that has seeped in.

The beginning of the discharge of around 540 Olympic swimming pools' worth of water over several decades is a big step in decommissioning the still highly dangerous site.

The release of the wastewater has been deemed safe by the International Atomic Energy Agency.

But China's foreign ministry said Thursday that "the ocean is the common property of all humanity, and forcibly starting the discharge of Fukushima's nuclear wastewater into the ocean... ignores international public interests".

Japan "did not prove the legitimacy" of the plan or the "long-term reliability of the nuclear wastewater purification equipment", it said.

Tokyo also "did not prove the authenticity and accuracy of the nuclear wastewater data, (and) did not prove that ocean discharge is harmless to the marine environment and human health".

"By treating the release of the wastewater as a fait accompli, the Japanese side has simultaneously placed itself in the international dock," the ministry added.

"The Japanese side should not cause secondary harm for local people or even the people of the world out of its own self-interest."

More than 10 arrested trying to enter Japan embassy in Seoul

More than 10 people were arrested Thursday for trying to enter the Japanese embassy in Seoul during a protest over the release of Fukushima wastewater, police told AFP.

"More than 10 people have been arrested for trying to breach the embassy," a police officer at the scene said.

A handful of protesters had gathered at the embassy, holding signs reading "The ocean is not Japan's waste bin" and "We oppose wastewater discharge."

The Yonhap News Agency said that 16 people -- all university students -- had been detained over the attempted break-in.

The students "attempted to enter the embassy office at around 1 pm (0400 GMT) while shouting slogans condemning the discharge of radioactive water," it reported.

"The police detained them on charges of trespassing and violating the Assembly and Demonstration Act."

All other protesters had been dispersed and police had restricted access to the building housing the embassy shortly after the incident, an AFP reporter saw.

Seoul has supported Tokyo's decision to release the treated wastewater from the crippled Fukushima nuclear plant, with Prime Minister Han Duck-soo saying Thursday there was "no need to be excessively concerned" about the plan.

Han also criticised what he called a "politically driven" campaign against the wastewater release, which was using "fake news" to fan fears.

There have been scattered public protests over the move, which is also staunchly opposed by the opposition Democratic Party.