Wild street performers were entertaining the enthusiastic crowds with their capers on the Royal Mile, the lively Saturday night street theatre edge of the vast Edinburgh Fringe. Yet literally within a step we were inside the relative calm and beauty of St Giles Cathedral for a more contemplative side of the same festival: Karl Jenkins’ The Armed Man: A Mass for Peace followed by Mozart’s Requiem. The programme, with its candlelit promise of thoughtful late-night spiritual meditation, delivered something quite different and surprising.

This is the 20th anniversary of the National Youth Choir of Scotland, grown and nurtured from a single choir on a summer school to 14 area choirs, national boys and girls choirs, a national training choir for the flagship National Youth Choir, or ‘Big NYCOS’ as it is affectionately known. Big NYCOS, fresh from a USA tour, has just returned from performing under John Eliot Gardiner at the BBC Proms, and will head off to France to repeat the concert. Indeed, down the road at the Usher Hall, the NYCOS Girls Choir was taking part in Bach’s St Matthew Passion with Gardiner conducting at the Edinburgh International Festival – an illustration of the summer of riches in the Festival City, and a youth choir organisation knitted completely into the mainstream musical fabric.

The Armed Man: A Mass for Peace by Karl Jenkins was written as a millennium commission for the Royal Armouries Museum in Leeds, looking back with regret at the wars in the 1900s and forward with hope for the next hundred years. It is a popular piece – both it and Jenkins appear in lists of ‘most performed works by living composers’, and a gift for a youth choir with its dancing tunes, catchy rhythms and darker side. Scored for a large orchestra with percussion, it was performed here by the NYCOS training choir conducted by Mark Evans with organ accompaniment by Christopher Nickol but with a drum, a trumpet and solo cello.

Evans drew an impressive variety of choral colouring from his 60 strong singers, dancing the French “L’Homme armé” folk-song to the fife and drum beat before a Muslim call to prayer from the pulpit, minaret style. Evans chose sensitive registration throughout, letting the singers create the drama. The 32ft rumble on the pedals introduced the Mass proper, with a polyphonic Kyrie with rounded singing, crystal clear words and even with a heavy balance of women’s voices, a pleasing blend. Men’s plainsong on biblical text introduced the Sanctus with difficult rhythms over drumbeats, skilfully done despite the building’s echo acoustic, Evans building drama, letting the voices soar. While the Latin Mass is central, its spirituality is amplified from the other texts, Kipling’s in the Hymn before Action and Dryden in Charge! ending in an ululating “Ah” and a deathly pause. The Last Post, trumpeted from the far end of the building, bounced its echoes off the ancient stone walls and columns, its universal lament suddenly bringing home the horrors of war.

The pedal-point setting of the powerful words of Toge Sankichi, a Hiroshima survivor, four giant discordant chords on the organ introducing Torches from the Mahabharata were both movingly sung by the choir with some fine solo work. Calm blended singing in the tread of the Agnus Dei and a rich mezzo solo on Guy Wilson’s Now the guns have stopped was followed by a warm Benedictus with cello solo. With the Mass done, the French folk song returned and the choral bells rang out a rousing carol before an unaccompanied hymn God shall wipe away all tears brought the work to a hopeful close in a genuinely moving performance.

Treated by most as a second half, but billed as a separate Fringe event, the main NYCOS choir under Christopher Bell performed Mozart’s Requiem in Robert Levin’s 1996 revision, again with Christopher Nickol at the organ. The lights dimmed and we all lit our candles. If we thought this would be a gentle late-night spiritual experience, we were mistaken as Bell went for an exciting full-on approach full of attack, vigour and sheer force. The sound was rich and absolutely glorious, filling St Giles wall to wall with soaring top lines. Walk-out solos and a semi-chorus blended together well in the quieter moments, but watching Bell urging his forces on was entertainment in itself, crouching down to control the singing and then walking backwards up a step to get greater height, using both arms in an upward diagonal swing to stoke the Amens – if you had handed him a shovel and red hot coals, it would not have been out of place.

This was a wonderful showcase for NYCOS centre stage in Edinburgh, with both events completely sold out. While Mozart impressed, the Armed Man had the spiritual edge.