Twenty years after it was founded, Carlo Petrini's protest against McDonald's has grown into an international movement that has revolutionised the way we eat and farm, not only in his native Italy but around the world. Peter Popham talks to the man his friends call the Volcano

Slow Food, the movement whose symbol is a small golden snail, is 20 years old next month. As symbols go, it's a good one. A snail is slow, and it is also food, the sort of food you are unlikely to find in any franchised outlet. Like a snail, Slow Food has gone round and round during its 20 years of life, always coming back to the subject of food: but each time in a drastically different way.

Like the lines on the shell of the snail, its development has traced an ever-expanding spiral. It began as a club of Italian gourmets, incandescent about the arrival of McDonald's in Rome's Piazza Spagna. Twenty years on, it is preparing to welcome 5,000 peasant food producers from every corner of the earth: cane rat farmers and rabbit breeders from Benin, saffron growers from Kashmir, pasta producers from Sicilian land confiscated from the Mafia, and champion cheese makers from the West Country.

It was Rome's first Golden Arches that spurred a restlessly self-reinventing journalist from Italy's far north-west into action. Carlo Petrini today is a grizzled, wiry man with a fuzz of grey beard, shortly to turn 57. He has a ready laugh and an affable manner and works from a small office in the town of Bra, where he was born and where he has lived all his life.

If you didn't know who he was, you would probably guess that he was a lover of folk music, a slightly left-wing, alternative sort of fellow with a weakness for Italian provincial cooking. All of which is true, of course. You wouldn't suppose him to be ambitious, and conventionally speaking that's right, too.

What you would never guess is that he is the head of an organisation with 83,000 members in 122 countries and still growing, and that once every two or three years something emerges from that affable, easy-going head to give the huge organisation he has founded another completely unforeseen spin.

In the process, he has done two things that practically no other Italian has achieved in the past half-century. He has created a movement which, while its origins are in Italy's Communist left, has succeeded in transcending all definitions of left and right, forging ties with politicians such as Gianni Alemanno, the "post- Fascist" Minister of Agriculture in Silvio Berlusconi's government. This refusal to bow to the shibboleths of Italian political tribalism makes Petrini deeply suspect to many on the left. He doesn't give a hoot.

The other thing he's done, as Mr Alemanno himself recently pointed out, is to create a movement which is "one of the very few things that Italy has launched that has a global dimension". Slow Food, the enemy of food globalisation, has itself become a global brand - and has managed this so successfully that many people with only a passing acquaintance with it have no idea that it is Italian.

In 1986, aged 37, "Carlin", as he is known, had already done a lot in life, but in an apparently haphazard, improvised fashion that did not suggest a person moving on to great things. He had campaigned for the legalisation of independent radio stations. He had worked as assistant to the mayor of his home town, and had displayed his talent for organisation by launching a local folk music festival, Cante' J'Euv, still an annual event. In 1977, he became a freelance restaurant reviewer, writing wine and food reviews for Il Manifesto, the independent Communist daily, and L'Unità, the other Communist daily, founded by Antonio Gramsci.

Then McDonald's came to the heart of Rome and Petrini got the jolt he needed. It was no longer enough to identify and celebrate wonderful Italian food and wine. Now it needed defending. On 26 and 27 July, Petrini and a handful of friends got together in the cellar of a notable restaurant called La Bella Rosin in the town of Fontanafredda to plot their counter-revolution, their fightback against McDonald's and all it represented. They formed themselves into an organisation, Arcigola, which was the direct antecedent of Slow Food. And already the wild notions were steaming from "Carlin's" brain.

"I first met him some 20 years ago," recalls Carlo Revello, a journalist. "Carlin talked about his projects, his trips to South America with a missionary friend. In the cellar in Fontanafredda, he [came up with] thousands of ideas, some of them so impracticable that they sounded more like science fiction ... At the time he struck me as a romantic dreamer, but in fact he has turned out to be an attentive and concrete observer. In these 20 years, Petrini the Volcano has realised thousands of dreams."

Three years of feverish brainstorming later, Arcigola had become Slow Food, and on 9 November 1989, at the Opera Comique in Paris, delegates from 15 countries endorsed the Slow Food Manifesto, written by Petrini and his friend, the journalist and poet Folco Portinari.

"Our century," it went, "first invented the machine, and then took it as its life model. We are enslaved by speed, and have all succumbed to the same insidious virus: Fast Life, which disrupts our habits, pervades the privacy of our homes, and forces us to eat Fast Foods. To be worthy of the name, Homo sapiens should rid himself of speed before it reduces him to a species in danger of extinction. A firm defence of quiet material pleasure is the only way to oppose the universal folly of Fast Life. May suitable doses of guaranteed sensual pleasure and slow, long-lasting enjoyment preserve us from the contagion of the multitude who mistake frenzy for efficiency." "A firm defence of quiet material pleasure ... suitable doses of guaranteed sensual pleasure ..." - this was less a call to arms than a call to the knife and fork, and the message was met with delirious joy wherever Carlin's evangelists went. And no wonder. You could eat long, tremendous meals washed down with Italy's best wines and consider yourself righteously on the barricades - keeping the "frenzied multitude" wolfing down their Big Macs at arm's length.

Twenty years on, the manifesto has worn less well than Karl Marx's Das Kapital. It sounds snobbish and brainlessly sybaritic, thin on analysis and concerned almost exclusively with the well-heeled diner's sense of comfort and well-being. But fortunately Carlin did not stop there.

At the outset, the whole emphasis of the movement was on the consumer, and on getting him or her to realise how much they lost when they gained a few minutes by eating Fast. Today, Petrini no longer accepts the term "consumer": he prefers "co-producer". "The term 'consumer' is not appropriate because it sanctions an injustice," he says. "The society of consumers means speed, over-production, waste ... The body of the consumer is an anxious body ... children are taught like battery chickens to be the consumers of the future. Teaching children not to want to be consumers is an extraordinary educational project."

"Slow Food's first battle," says the journalist Luciana Castellina, "was against the disturbing modernity of the food industry, during the period in which people were thinking that food could be substituted by pills. Slow Food succeeded in impressing the idea that the act of eating was a ritualised space, a connotation of the collectivity."

The movement spread the idea around the world through the Convivium, the nice Latin word from which we derive "convivial" and which Carlin gave to the movement's branches. Convivia sprang up all over the globe; they now number around 800, and while they began as associations of gourmets, their meaning developed as Slow Food's ambitions grew and changed. Today they build relationships with producers, campaign to protect traditional foods, encourage cooks to use local foods - never forgetting that the appreciation of pleasure and quality is at the root of what they do.

Because Petrini realised soon enough that keeping diners wise was only the beginning of the struggle. The modern food industry had contaminated the whole river. Traditional food producers were going out of business in their thousands, traditional food was becoming extinct, grocers were being closed down by the supermarkets, in the background the entire environment of the earth was being polluted to a degree that put the pleasures of a good dinner into perspective. "There is no pleasure," Petrini said last week, "without justice and without the cleansing of the environment."

Petrini's initiatives over the past five years have been a solid riposte to his critics. In a complex of period buildings in the town of Pollenzo, he has overseen the creation of the world's first University of Gastronomy, where food and wine are taught and studied in all their aspects. And then there is the "world meeting of food communities", the 5,000 beekeepers and fishermen and cane rat breeders pouring into Turin in October, two years after the first of these grand "Terra Madre" ("Mother Earth") events. This time they will interact with teachers from Pollenzo's university, and 1,000 chefs.

"It will be a dialogue between farmers, university teachers and cooks," Petrini told me, "with the idea of reinforcing small-scale economies and helping agricultural production to become rooted again in local areas. The concrete results will emerge when they get home. The meeting will be a stimulus, and will lead to the creation of a network between them all. Between leaving Turin and meeting up there again in 2008, this network will enable them to share knowledge, wisdom and information."

Among the participants will be more than 250 producers from Italy, which is still the great kitchen of the movement, and the place where its achievements are most tangible. The agriculture of Tuscany, for example, has been turned round in the past 20 years, substantially thanks to the energy and the propaganda of Slow Food. "Twenty years ago," says Maria Grazia Mammuccini, president of Tuscany's agency for development and innovation in agriculture, "the model being followed in agriculture here was industrial. But it was no good for Tuscany because the farms are too small and there are no large plain areas, and 96 per cent of farms are family-run. So instead we encouraged small producers to understand the value of the typical local produce, and now many of them don't just grow grapes or olives but produce their own wine and oil of high quality and sell it. Slow Food has been vital in getting the public to understand the importance of this. And in these 20 years Slow Food itself has changed."