SHIZUOKA: The world's longest-serving death row prisoner was acquitted on Thursday, more than half a century after his murder conviction, when a Japanese court ruled that evidence had been fabricated.

Ailing health prevented 88-year-old former boxer Iwao Hakamada from being in the court to learn the outcome of his retrial, which was granted a decade ago after a long campaign by supporters.

But his 91-year-old sister Hideko, who often speaks for him, bowed deeply to the Shizuoka District Court judge who declared Hakamada innocent.

Hakamada spent 46 years on death row after being convicted in 1968 of killing his boss, the man's wife and their two teenage children.

"Investigators tampered with clothes by getting blood on them" which they then hid in a tank of miso, or fermented soybean paste, said Thursday's ruling, seen by AFP.

It also slammed the use of "inhumane interrogations meant to force a statement... by imposing mental and physical pain".

"The prosecution's records were obtained by effectively infringing on the defendant's right to remain silent, under circumstances extremely likely to elicit a false confession," the ruling said.

Hundreds of people queued in the morning to try to secure a seat for the verdict in the murder saga that has gripped the nation and sparked scrutiny of Japan's justice system.

"I went to the prosecutors' office and said, 'don't force us to fight until I turn 100'," Hideko told reporters before the verdict.

Hideko wore a white jacket and, asked if it symbolised her brother's innocence, said she had deliberately avoided dark colours.

'A bout every day'

Japan is the only major industrialised democracy other than the United States to retain capital punishment, a policy that has broad public support.

Hakamada is the fifth death row inmate granted a retrial in Japan's post-war history. All four previous cases also resulted in exonerations.

His lead lawyer Hideyo Ogawa said Hakamada sometimes seems like he "lives in a world of fantasy" after decades of detention, mostly in solitary confinement.

Describing his battle to obtain an acquittal to AFP in 2018, Hakamada said he felt he was "fighting a bout every day".

"Once you think you can't win, there is no path to victory," he said.

Hakamada appeared not to be aware of the decision yet after Japanese media reported that supporters had removed the batteries from his television remote control on Thursday so he could not watch the verdict live.

Hideko told reporters before the hearing she wanted to tell him the news soon, but at the right moment.

He was filmed shortly after the decision leaving home to go for a walk, dressed in a short-sleeved shirt and a green hat.

'Hostage justice'

The Supreme Court upheld Hakamada's death sentence in 1980 but his supporters fought for decades to have the case reopened.

A turning point came in 2014 when a retrial was granted and Hakamada was released from prison. However, legal wrangling, including a pushback by prosecutors, meant it took until last year for the retrial to begin.

"For so long, we have fought a battle that has felt endless," Hideko told reporters in July.

Hakamada initially denied having robbed and murdered the victims in 1966, but then confessed following what he later described as a brutal police interrogation that included beatings.

Supporter Atsushi Zukeran, wearing a T-shirt saying "Free Hakamada Now", said outside the court the case was "a painful reminder of how Japan's criminal justice system must change".

Given how long the affair dragged on, "part of me wouldn't be able to celebrate the acquittal entirely", Zukeran said.

Hakamada's case is "just one of countless examples of Japan's so-called 'hostage justice' system", Teppei Kasai, Asia programme officer for Human Rights Watch, told AFP ahead of the verdict.

Amnesty International said it was "overjoyed" by the outcome.

"After enduring almost half a century of wrongful imprisonment and a further 10 years waiting for his retrial, this verdict is an important recognition of the profound injustice he endured for most of his life," the group's East Asia Researcher Boram Jang said in a statement.