ONOMICHI (JAPAN) - ON A rainy weekday at Onomichi prison, dozens of inmates are hunched over rows of desks and starting to tire.

What they are doing is not exactly hard labour: making casings for light bulbs, tying lengths of wire to luggage tags and making pairs of knitted slippers.

The silence is broken only when a guard enters the room, salutes and barks a greeting to a colleague. The show of authority seems out of place: Few of the prisoners, if any, have the energy to start a riot, let alone attempt to escape.

There may be no hardened yakuza gangsters here, but Onomichi's 70 or so ageing inmates nonetheless represent the biggest challenge facing Japan's criminal justice system in decades, a British newspaper has reported.

While other developed countries debate what to do with youth offenders, Japan is grappling with a different kind of crime wave, and one that demographic trends show can only get worse.

The over-60s are the fastest-growing criminal group in Japan. Citing figures from the national police agency, The Guardian says the number of Japanese aged 70 and over charged with crimes trebled between 2000 and 2006 - from 9,478 to 28,892.

Last year, elderly men and women were responsible for almost one in seven recorded crimes, compared with one in 25 a decade earlier. While most of them were guilty of theft, shoplifting and other petty offences, more than 150 were charged with murder.

In the years to come, many of Japan's 74 prisons will end up looking like Onomichi, a prison about 640km south-west of Tokyo that first started catering to older prisoners 20 years ago. The prison has incarcerated just over 300 offenders, 76 of whom are aged 65 or older. The average age of the men in the special ward is almost 70; the oldest man is 89.

Almost all are serving sentences of one to several years for theft - usually of food from supermarkets; small-time fraud; and, in a few cases, possession of drugs. The most serious offender, a man in his 70s, is serving 10 years for murder.

Almost all of Onomichi's elderly inmates have their own cells. Charts on their cell doors stipulate special dietary requirements and medication regimes. A handrail runs the length of the corridor, and makeshift wheelchair ramps are kept at the entrance to the communal baths.

The rise of the superannuated criminal is only partly explained by Japan's rapidly ageing population. While the number of Japanese aged 60 and over grew by 17per cent between 2000 and 2006, the number of prisoners in the same age group soared by 87per cent.

Criminologists put the blame on record levels of poverty among pensioners, the breakdown of the extended family, and a lack of professional help for those suffering from depression and other mental illnesses.

Life on the outside can be unforgiving for elderly men with a criminal record, reports The Guardian. For many, the best chance of security, decent health care and three meals a day is another stint behind bars. Almost two-thirds of Onomichi's older inmates will return within five years of their release.

A 76-year-old inmate told a Japanese newspaper: 'I have clothing, food and housing and I'm taken care of when I get sick. Prison life is like a strict nursing home.'