Japan began releasing wastewater from the crippled Fukushima nuclear plant Thursday in an operation it insists is safe but has generated a fierce backlash from China.

The beginning of the discharge of around 540 Olympic swimming pools' worth of water into the Pacific 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 engineers behind computer screens 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.

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.

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