The hypnotic strains of Oriental gongs, xylophones and drums emanated from the stage as 16 musicians in a traditional Indonesian percussion orchestra sat cross-legged, swaying to the beat.
The ensemble was playing the Javanese "gamelan" - 20 bronze percussion instruments which date back 1,400 years.
But these were no ordinary musicians: they were inmates of Brixton Prison, playing compositions with titles such as "Bestowing Honour" and "Broken Drum" - some mixed with personally written rap lyrics - for an audience of fellow prisoners and guards.
Their efforts at playing the gamelan, on loan from the Royal Festival Hall, will be broadcast by Radio 3 next month and the prison ensemble is in the process of making a CD.
The initiative, which is aimed at improving the personal life skills of inmates through music workshops, was created by Cathy Eastburn, director of the Good Vibrations project. She said the gamelan was particularly therapeutic because it forced a group to communicate and work as a team, and it had a soothing, resonant sound.
"It's a completely communal activity. Every part of the percussion is equally important. Through it, you learn how to respect everyone else," she said.
John Podmore, the prison's governor, said it was not just an exercise in keeping inmates occupied but an essential part of rehabilitation. "This is all about reducing the rate of re-offending, about helping the prisoners to develop self-esteem, allowing them to work in groups and equip them with communication skills," he said.
He dismissed the idea it was a "treat for cheats" and said the money had been raised through the charity sector as well as some funds from the Arts Council.
"What's out there, being done in the community, as the gamelan is in places like the South Bank in London, should be brought in if it's a positive force of good," said Mr Podmore.
None of the 16 inmates who had signed up for the crash course had heard of the gamelan before and were astonished by the large-scale instruments when they first saw them.
Most agreed that the achievement of mastering the instruments was not just about the music.
Francis Howe, 26, from Northern Ireland, who is serving a seven-year sentence for drug dealing, said his desire to be part of the band was to open himself up to new influences.
"I'm in here for importing drugs and I'd never talked to drug addicts until I talked to some of the men doing the gamelan.
"I come from a big criminal family in Northern Ireland and this kind of thing opens you up to change. I wanted to open my mind up to a different culture and it's given me a new opportunity in life. I feel like taking gamelan back to the community in west Belfast and teaching them. A lot can be done through music," he said.
Nick Chernikeeff, 47, who is serving a life sentence for two grievous bodily harm incidents, said the music had pulled him out of a depression which he had sunk into during the first few years of prison life. "I had the need to find something positive. Unfortunately, I'm a lifer and I got very despondent when I first came here. This music takes you out of prison. It brings people together and strangers become friends," he said.
One 26-year-old inmate facing trial for conspiracy to murder, who spoke of the monotony of prison life, said the gamelan had had a deeply soothing effect on him and other band members. "It's just so mellow. You get into a kind of trance when you play it. It's like being in another world. At first it felt like a weird thing to do. I'd never seen the instruments and I thought, 'what's all this about?'
"I was embarrassed to take my shoes off to play it. But it's about having respect for the instrument. It's a good thing," he said.
A prisoner known as Gamma who rapped to one gamelan composition, said the instrument had been easy to pick up and described its sound as a "spiritual experience".
The ensemble's music tutor, John Pawson, said that while some men had resisted the gamelan's charms at first, they were quickly won over.
"It has an ability to sound good from the moment you begin playing it. And it's a musical instrument that very few people know about so there's no cultural baggage. Everyone is coming to it for the first time. It's a great equaliser."
n The Brixton Prison ensemble's Gamelan sounds will feature on Radio 3's World Routes programme on 8 October.Reuse content