Govinda Prasad Mainali, of Nepal, gestures during a press conference in Kathmandu, Nepal.

Tokyo - A Nepalese migrant worker who spent 15 years in a Japanese jail for a murder he did not commit claims he was tortured while incarcerated and has urged Tokyo to reform its police and prison system.

Govinda Prasad Mainali was waiting tables in an Indian restaurant in the Japanese capital, sending money home to his family in Kathmandu, when he was arrested on suspicion of strangling Yasuko Watanabe in 1997.

Watanabe had been an up-and-coming economist earning a comfortable salary and yet was reportedly living a double life as a sex worker who would walk Tokyo's streets at night.

When her body was found in a sleazy downtown apartment, Mainali was charged with her murder and handed a life sentence.

In a scandal which has brought ignominy on Japan's justice system and captivated Tokyo society, Mainali was acquitted in a retrial on Wednesday after 15 years in jail, as DNA evidence proved he could not have committed the crime.

“I was forced to undergo 15 years of horrible and torturous time in jail despite being innocent,” Mainali told reporters in Kathmandu after his acquittal.

“Had the DNA test not been conducted, I would have been languishing in jail and probably would have died there.”

Mainali, now 46, was deported to Nepal in June after his conviction was quashed and he was finally declared not guilty by the Tokyo High Court five months later.

The case has attracted lurid headlines and exposed Japan's justice system to heavy criticism by rights campaigners.

Watanabe, who was 39 when she died, grew up in an affluent Tokyo neighbourhood, majored in economics and followed in the footsteps of her late father into the Tokyo Electric Power utility company.

She was a high-flyer but became frustrated as she felt she was passed over for promotions, author Shinichi Sano said in a best-seller book on the killing.

She began to work at night in a hostess bar, before turning to high-class prostitution.

But she would also walk the streets, soliciting for as little as $20, and this is how she met Mainali, his original trial was told.

Mainali was charged after traces of his sperm were found in a condom at the murder scene and the court found him guilty in 2000 of killing Watanabe.

The Supreme Court upheld his life sentence in 2003.

Mainali, who had always maintained his innocence, was released and granted a retrial after fresh DNA evidence proved the original probe had overlooked the fact that semen found inside Watanabe was not Mainali's.

The semen in the condom sample was also dated to well before her death.

Leading rights groups and the United Nations have criticised Japan's daiyo kangoku - or substitute prison - system, which allows suspects to be detained for up to 23 days without charge and with limited access to lawyers.

Almost all murder cases in Japan result in guilty verdicts because so many are based on confessions.

Amnesty International says Mainali was denied access to lawyers after his arrest and was beaten, kicked and pinned against a wall by police officers.

“Mainali's acquittal shines a spotlight not only on the injustice he has suffered but on a system which, unless reformed, will perpetuate violations of international fair trial standards,” its Japan spokesman Rajiv Narayan said.

Looking nervous and flanked by family members as he read out a statement in Nepali and answered questions in fluent Japanese, Mainali said he had not yet decided whether to seek compensation.

“The details on torture and my experience of jail life will come out in a book very soon. It's a weakness of Japan's legal system. The police failed to conduct a proper investigation,” he said.

Mainali said he felt “tears of joy swelling in my eyes” when he learned of his acquittal.

Mainali has missed out on his daughters Mithila, 20, and Alisha, 19, growing up and has been unable to look after his 80-year-old mother or celebrate wedding anniversaries with his wife, Radha.

His family say he is anxious about venturing out of his home after so long in jail.

“The DNA tests should have been conducted 15 years ago, not this late. My brother lost the prime time of his life behind the bars. We all suffered a lot from the wrongful conviction,” Indra Prasad told reporters. - AFP