The sight is awesome. Three thousand people, hand in hand, dance trance-like in a labyrinthine spiral to the singing of two elderly men, who look a bit like farmers from Co Mayo in their tweed caps.

Henri and Jann Morvan, who are sweating it out in the heat on a podium in the middle of the hall, have been singing kan ha kiskan, the distinctive form of call-and-response singing for the Fest Noz, or Breton ceili, since childhood. Now in their late 60s, they have almost mythic status among the Breton dancing community which, judging from the numbers enthusiastically joining in during the Interceltique Festival in Lorient last week, must be huge.

Although the Morvan brothers may be veterans, the dancers are mostly in their late teens and early 20s, and respond just as enthusiastically to the music of Skolvan, a contemporary group incorporating jazz, world and techno influences.

During a 10-day Celtophiles' festival, the compelling and strangely moving sight of long rows of people moving to the music in that dignified inclusive manner that is the way of Breton dance was as impressive as the sound of the pipes.

The annual Festival Interceltique de Lorient, on the southern coast of Brittany, reached its grand finale on Sunday night with a stunning outdoor spectacle. This included Donal Lunny and Nollaig Casey, virtuoso Galician piper Carlos Nunez and Scottish singer Karen Matheson.

It is now one of the largest festivals in Europe, attracting almost half a million visitors. And amazingly, it is run mostly on voluntary labour, with hundreds helping out for the love of Celtic music, the craic and a couple of free beer tickets.

During the first full week in August the otherwise sleepy Breton town is transformed into "Planet Celt". What began 30 years ago as a pipe band competition and a dream of cultural recognition has mushroomed into a cornucopia of all things Celtic, not just music, dance, instruments and costumes, but also food, art, literature and - of late - lots of dot coms.

But the Celts went global some time ago. We have all noticed a certain familiarity about bush bands in Australia, bluegrass in the Appalachians and Cape Breton fiddlers. The Celts have always been wanderers, either by choice or by force.

So in this era of world music and global villages, it should come as no surprise to find that lost Celtic cousins from Italy, Romania and Argentina are rediscovering their roots and want to stand shoulder to shoulder with the Scots, Irish, Welsh, Cornish, Bretons, Manx, Galicians and Asturians.

This year they have branched out further, with three Internet sites featuring the festival. The Celtic connection is a marketing man's dream, particularly in the burgeoning world of e-commerce and niche marketing.

Such cynicism is faced sanguinely by Tomas Mac Ruairi, uachtar an Chonradh na Gaeilge, the man behind the 220-strong Irish delegation at the festival.

"Yes, I think it's inevitable that the festival has become more commercial, that's just a fact of modern life." What can we do about it? "Make sure that the groups who come to the festival are of the highest quality, firmly rooted in the culture and, as much as possible, in the language."

This year's Irish contingent included headline acts such as the Chieftains and Sharon Shannon, virtuoso soloists piper Meaiti Jo Seamus and singer Aine Ni Dhroighneain from Connemara, pipe bands from Arklow, Armagh, Carrigaline, a dance troupe from Dublin, traditional groups from Clondalkin, Cork and, believe it or not, Rome, as well as teachers for a range of workshops.

Strangely enough, although there are classes in the Irish fiddle and uileann pipes at the festival, there is no tuition in Breton instrumental music. In many ways the Irish have an influence disproportionate to their numbers at the festival, and indeed within Celtic culture in general.

This is particularly true of the music scene, where the Irish are seen as market-leaders and where jigs and reels dominate the sessions, even if three-quarters of the musicians are Breton.

That may all change soon. The festival has had an important role in forging a sense of Celtic identity and popularising Celtic music. It has boosted Irish music and now seems to be offering new possibilities for Breton music, with prominent performers such as Eileen Ivers incorporating Breton material into her repertoire; artists such as pianist Didier Squiban becoming sell-out acts in Germany; an increase in the popularity of Breton dance, a growth in Breton language primary schools and the introduction of TV Briez, the Breton-language station which starts broadcasting on September 1st.

The festival may now be on the verge of becoming a victim of its own success. Some local people are none too happy about the disruption caused by the crowds. Complaints about noise, traffic and the extent to which the centre of the town is taken over by the event have left the festival organisers with some headaches.