World Thailand Moo, 23, builds muscle for Muay Thai on the Klong Prem prison yard, in Bangkok, Nov. 2013. Aaron Joel Santos

Moo doesnt seem like someone with 23 years of a 24-year sentence left to serve. His career as a driver for a drugs gang ended when he was nabbed piloting a carload of yaba, a methamphetamine derivative that translates literally as crazy drug. Now the 22-year-old spends 13 hours each day with four other men confined along to a 1.5 x 3.5 meter cell. I made a big mistake, says Moo. And Ive paid the price for what I did.

Yet Moo is remarkably upbeat when we meet him in Bangkoks high security Klong Prem Prison, notoriously dubbed the Bangkok Hilton by overseas residents. His levity is due in part to a unique quirk in the Thai correctional system that allows inmates to use their pugilistic potency to shave years off their sentence. From now on I only want to do things that can enhance my life such as using my boxing skills, Moo says.

Muay Thai (Thai kickboxing) tournaments have been a fixture of prison regimes in the Southeast Asian nation for centuries. The tradition of holding bouts behind bars started in 1767 when the Burmese took thousands of Thai soldiers prisoner after the downfall of Thailands then capital Ayutthaya. While incarcerated, the best Thai boxers were coerced into fighting Burmese combat experts. The ultimate champion, as legend tells it, was a Thai named Nai Khanomtom, who was granted his freedom after besting one of the Burmese monarchs top fighters.

This ancient tradition of favoring prisoners skilled in the ring is alive and well in Klong Prem and other Thai prisons. But while inmates fight literally for their freedom, international Muay Thai fighters from around the world are now flocking to enjoy a truly unique experience, as well as earn peerless bragging rights.

It was a special feeling to test my Muay Thai skills out in a Thai prison, says Alexei Vignol, a 20-year-old from Lyon in France who was narrowly beaten in Klong Prem recently. I guess you could say that it is intimidating to fight criminals inside a high security facility, but if you get scared by that then you shouldnt be a boxer.

The incorporation of foreigners has been spearheaded by an independent organization named Prison Fight. Billing itself as a charity, Prison Fight provides sport equipment, modest financial rewards inmates winnings are squirrelled away in a prison account and used to provide for their families and, most importantly, offers the successful a realistic chance at getting their sentences reduced.

We do this to give the fighters an experience and also to give something back to the country, adds Kiril Sokur, the Estonian businessman behind the Prison Fight enterprise. Giving these guys [the prisoners] the opportunity to prove their talents is important. They may be criminals but they are also human beings so staging the fights is good karma for us.

During a recent visit to Klong Prem, we encountered an atmosphere that was surprisingly convivial. As training took place in the ring set up in the prison yard, prisoners clowned around with each other, shadow boxing and dishing out playful clouts. One wiry, grinning inmate with intricate tattoos covering his entire torso, was a particularly energetic presence. Thats Chui, said Nikki. Hes a hitman.

Section 5 is for Klong Prems lifers. It is also the unofficial Muay Thai wing of the prison. Criminals with previous boxing experience like Moo and his cellmates are placed here upon entering Klong Prem. Guards, meanwhile, scout out candidates from other wings for a possible transfer. A long, wide central corridor that separates the cells is used for twice-daily hour-long running sessions. A boxing ring and rudimentary training equipment dominates the outside yard.

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Prison Inmates in Thailand Fight Foreigners for Their Freedom