The three men were senior officials at the TEPCO firm operating the Fukushima Daiichi plant and had faced up to five years in prison if convicted.

"All defendants are not guilty," the presiding judge announced to the Tokyo courtroom, where a woman in the gallery could be heard saying "unbelievable" as the verdict was pronounced.

The decision is likely to be appealed, extending the legal wrangling over responsibility for the disaster, more than eight years after the worst nuclear accident since Chernobyl.

Outside the courtroom, dozens of people staged a rally, including some who had travelled from the Fukushima region to hear the verdict.

"It is absolutely an unjust ruling. We absolutely cannot accept this," one woman said angrily, addressing the crowd.

"We will appeal this and continue our fight," shouted a man nearby.

The three former executives were accused of professional negligence resulting in death and injury for failing to act on information about the risks from a major tsunami, but they argued the data available to them at the time was unreliable.

The presiding judge said the verdict had turned on the "predictability" of the massive tsunami that swamped the nuclear plant in March 2011 after a 9.0-magnitude undersea earthquake.

- Complex path to trial -

No one was killed in the nuclear meltdown, but the tsunami left 18,500 dead or missing.

The ex-TEPCO executives faced trial in relation to the deaths of more than 40 hospitalised patients who died after having to be evacuated following the nuclear disaster.

Prosecutors twice declined to proceed with the case, citing insufficient evidence and a slim chance of conviction, but were forced to proceed after a judicial review panel composed of ordinary citizens ruled that the trio should face trial.

All three defendants -- former TEPCO chairman Tsunehisa Katsumata, 79, and former vice presidents Sakae Muto, 69, and Ichiro Takekuro, 73 -- had pleaded not guilty.

The prosecutors said the men were present at meetings where experts warned of the anticipated height of a tsunami off the Fukushima coast and should have taken better safety measures.

They argued the executives were presented data warning a tsunami exceeding 10 metres (33 feet) could trigger power loss and a major disaster at the plant.

And a TEPCO internal study, based on a government report, concluded that a wave of up to 15.7 metres could hit after a magnitude-8.3 quake.

In the event, when a 9.0-magnitude quake hit offshore on March 11, 2011, waves as high as 14 metres swamped the reactors' cooling systems.

The resulting meltdown forced massive evacuations and left parts of the surrounding area uninhabitable -- in some cases possibly forever.

- 'No one taking responsibility' -

The three defendants have apologised, but argued they could not have foreseen the disaster based on the available evidence and that they thought officials in the firm responsible for nuclear safety had taken appropriate measures.

"It is difficult to deal with issues that are uncertain and obscure," Takekuro said during the trial.

Separately from the criminal case, dozens of civil lawsuits have been filed against the government and TEPCO.

Some district courts have granted damages to local residents, ordering TEPCO and the government to pay.

Before the verdict, protestors outside the court said the trial was a chance to hold someone accountable for the disaster.

"If we don't hear guilty verdicts, our years-long efforts to bring this to court will not have been rewarded," said Saki Okawara, 67, who came from Miharu in the Fukushima region to hear the verdict.

"And Japanese society's culture of no one taking responsibility will continue."

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TEPCO - TOKYO ELECTRIC POWER