A fierce row over a breach of imperial etiquette has gripped Japan but commentators say the protocol slip is a convenient excuse to attack someone who dares to speak out.

Actor-turned-politician Taro Yamamoto, who was elected to parliament as an independent on a strongly anti-nuclear platform, caused outrage by handing a letter to Emperor Akihito during a royal garden party on October 31.

Yamamoto, 38, said the hand-written note had been his attempt to let the emperor know directly about the plight of people affected by the atomic disaster at Fukushima.

He said he feared the figurehead was being shielded by his entourage from the truth about the extent of suffering more than two-and-a-half years after a huge tsunami smashed into the plant, causing meltdowns that forced tens of thousands from their homes.

The former actor -- who claims his stage and screen work dried up when he started speaking out against nuclear power -- later apologised and admitted that his actions were "inappropriate".

He told a press conference he was desperate to draw the emperor's attention to the lingering crisis at Fukushima.

But the apology did not appease lawmakers, particularly conservative veterans in the ruling Liberal Democratic Party (LDP), who said the political novice had committed the unforgivable sin of dragging the revered emperor into the political fray.

The Japanese press corps has hounded Yamamoto amid growing calls for his ouster.

"He should resign. That is how bad this is," deeply conservative education minister Hakubun Shimomura said.

"It was nothing but politicising the emperor," he added.

So far Yamamoto has held his nerve and refused to resign, despite being censured by the speaker of the lower house and banned from attending any more palace events.

The charge of politicising the emperor is a serious one in Japan, where memories linger of the disastrous and brutal warring carried out in the name of his father -- Hirohito -- in the first half of the 20th century.

Hirohito was regarded by his subjects as semi-divine. He was invoked by frontline troops and kamikaze pilots in World War II as they went off to their deaths confident in the righteousness and holiness of their cause.

Despite the horrific crimes committed in his name -- and, critics say, with his connivance and blessing -- US occupiers opted to keep Hirohito on the throne.

He was forced to renounce his divinity and the role was transformed into that of a figurehead standing above the mudslinging of daily governance and as a symbol of the nation.

Freelance journalist Rei Shiva said the vitriol since Yamamoto's stunt illustrates lingering traditional views in Japanese society.

"Some people in this country still consider the emperor to be sacred," he told AFP.

The actual issue Yamamoto wanted to highlight -- the plight of children and nuclear workers in a country where power firms, regulators and government have been criticised for a cosy and complicitous relationship -- has been buried in favour of an argument over form.

"What he did shouldn't be regarded as rude, but the media have ganged up against him," Shiva said.

"Japan is a democratic country, where sovereignty resides with the people. We no longer have a lese-majeste law," he added.

The charge of disrespecting the emperor is just a stick with which to beat Yamamoto, said Shinji Yamashita, a journalist specialising in royal matters -- a way to get at someone establishment figures regard as an uppity young man.

"I wonder what would have happened if it had been an LDP parliamentarian member who did it," said Yamashita, a former official of the Imperial Household Agency.

"I think what he did was about politicising the emperor -- and above all, it was impolite," he told AFP. "But it wasn't anything to make such a fuss about.

"Many people didn't like him in the first place, and this has given the media a reason to gang up on him."