For the next two weeks, projected light will be Nicolas Tobazéon Chevalier's paint and the buildings and vegetation of Galway his canvas.

Visitors to Galway in the next couple of weeks will have a special surprise after dark every night, when French artist Nicolas Tobazéon Chevalier's Projections Monumentales invade the city as part of the annual arts festival, which starts on Monday. At Eyre Square, one of Tobazéon Chevalier's light sculptures will quiver in the branches of a big horse-chestnut tree.

"Vegetation is très sympa because it conveys a sense of movement, but you have to be directly in the axis of the projector to see the image - otherwise you see only light on the leaves," he explains.

Flat walls - like the side of the OPW building next to the Great Southern Hotel on Eyre Square - move in another way. "The passer-by plays a role," Tobazéon Chevalier continues. "As he changes his angle of vision, according to his position, the image gets bigger or shrinks."

Walking down Williamsgate Street to the corner of Eglinton Street, the evening stroller will see how the yellow-gold walls of Brown Thomas department store lend warmth to the Frenchman's creations.

"When shows and theatre are over and people spill into the streets, Galway will be like a canvas before them, extending into yet another event," predicts Paul Fahy, the festival's publicity manager.

There will be light sculptures of fiddlers and a saxophone player, beer drinkers, portraits and self-portraits. The fiddler is a memory from Tobazéon Chevalier's first childhood visit to Ireland with his parents. When he scouted Galway for suitable sites in April, the west was enjoying a warm spell.

"It had an almost Mediterranean feel to it," he says. "The colours are very bright - red, blue, green buildings; the pedestrian zone, where I saw a pianist playing outside. I was there for four days and I worked in my shirtsleeves." He liked the sloping roofs of many Galway buildings, and intends to use some as visual "hats" for his personages.

Down Eglinton Street through Francis Street to Newtownsmith, the Courthouse is also being transformed by Tobazéon Chevalier's imagination. Along the River Corrib, there'll be ephemeral artworks on the old Hygeia building and the side of Bridge Mills. From Wolfe Tone Bridge, turn left towards Quay Street to find a face on the back of Port More apartments in front of Nimmo's Wine Bar. The Halls office block on Quay Street will be "painted" with light, and there will also be an image on Lynch's Castle. Down on the docks, Tobazéon Chevalier is eager to see how his work will reflect on the metallic surface of oil storage tanks.

Tobazéon Chevalier has been told his images look aboriginal, African, Greek, West Indian, pre-Columbian, Oceanic, or like Picasso's Cubist paintings. He was strongly influenced by the early Christian Romanesque art his father loves.

"Where do they come from? I'd like to know myself," he laughs. "They come from inside me. These images are universal forms that belong to mankind."

The 39-year-old artist was born in Avignon, the son of a primary school teacher and a poet-turned-insurance agent. His parents had a quixotic streak; before he was born, they struggled for a while in a back-to-the-land peasants' co-operative.

When he was six, Tobazéon Chevalier's mother hired a private tutor to teach him engraving, painting and art history. It was in memory of his mother that the artist began using her name after she died a decade ago. "Tobazéon sounds more exotic than just Chevalier," he laughs. The name came from his maternal grandfather, but since the grandfather was a foundling, he's not sure of its origin.

Tobazéon Chevalier studied fine art in Barcelona, Aix-en-Provence and Avignon, but he found it difficult to make a living and so trained as a stage manager. One evening in the early 1990s, an assistant director asked him to slip a "gobo" - a glass disc that "paints" a beam of light, or the panes of a window, or prison bars, on a stage set - into the theatre projector. It was a moment of revelation.

"I realised what I could do the second I saw it," he recalls. The Projections Monumentales were born. Drawing on his experience of etching and engraving, Tobazéon Chevalier prepared the small glass stencils, about six centimetres in diameter. "What really blows me away is the contrast," he says. "I work in miniature, with a miner's lamp on my forehead and a magnifying glass - like a surgeon or a goldsmith. And the result is the size of a building."

His most ambitious project was a farewell tribute to the Cité Apollinaire, a housing estate on the Avignon ring road.

"They were about to demolish the building," he says. "I'd had my artist's studio there for 10 years. It was a neglected area, like most of the quartiers sensibles [sensitive neighbourhoods] in France - a lot of immigrants, lifts that don't work, uncollected rubbish that smells bad. Four-fifths of the flats were empty, so, with friends, we negotiated to use them for free, paying only maintenance charges. A hundred artists went through there in 10 years."

Tobazéon Chevalier amassed €18,000 to finance the project, and persuaded the city of Avignon to let him install projectors on the 14th-storey roof of the social security building. "They were a little nervous, because if we'd blown the electrical fuses, it would have messed up payments for 200,000 people," he says. By projecting human figures the length of the high-rise buildings, Tobazéon Chevalier made a political statement.

"Most of the people in the Cité Apollinaire were north African, and there is a lot of racism in southern France. By creating an artwork there, we displaced the centre of interest from museums, where you have to pay an entry fee; we brought art out to the ring road," he says.

Tobazéon Chevalier describes himself as "a street artist, so that people can have free, direct access to art". Contemporary artists, he says, "have a problem with the purpose and destination of what they're doing. Is it to hang in a gallery, so people with money can take it home?" He also believes that art should be ephemeral. At the age of 20, he and some friends hid inside the Paris metro overnight, to glue their three-by-four-metre posters over advertising hoardings. "They'd last for a day or two," he says.

Starting in 2000, he directed an 18-month project in Avignon called A l'affiche, in which 80 artists exhibited posters on hoardings. The posters were covered up with new art every three weeks.

Having mastered the mysteries of "gobos", electrical generators and halogen beams, Tobazéon Chevalier is experimenting with colour and movement by using multiple projectors. One cherished project is to "paint" the chimneys of a French nuclear power plant, putting images on the smoke as well as the cement towers. He's still seeking authorisation from the French authorities.

He has discussed his other dream with Afghan embassy officials and archaeologists - to recreate in situ the Bamiyan Buddhas that the Taliban destroyed. "It's fun having crazy ideas," he says. "But it's even better to realise them."

So far, Tobazéon Chevalier's interlocutors balk at the thought of bringing contemporary art to Afghanistan. Like the Cité Apollinaire, it would be a question of "displacing art from the centre".

"If westerners want to see the Buddhas, they'd have to go to Afghanistan," he explains. In the meantime, they can contemplate giant fiddlers, beer drinkers and portraits on the walls of Galway.

Nicolas Tobazéon Chevalier's Projections Monumentales runs nightly throughout the festival, from Monday to July 28th. Details at www.galwayartsfestival.ie