Ah, the smell of freshly cut grass and the faint whiff of controversy

High witness: They came (those who remembered to), they marched (well, shuffled) and they smoked. Man did they smoke.

It began beside the grey block war memorial to the London Borough of Lambeth's finest, who had given their lives fighting the Kaiser, and it ended up two miles away in a big open space with thousands of new age visionaries, trance-state guitar soloists, pathologically earnest students, the occasional marauding hound off a leash, a fair slice of what used to be called Middle England, and lots – and lots – of cannabis.

If ever a short march could trace the possible trajectory of 21st-century Britain, the 2003 Cannabis March was it.

Like most organised offensives aimed at the heart of the state machine, it began as an assembly of small groups of five or six scattered throughout Kennington Park, small knots of denim and leather arranged in earnest, inward-facing circles. They could have been revolutionaries talking quietly of freedom. A longer look, however, exposed the particular variety of freedom on offer today: sick greyish blooms of smoke, and the rich, candy-floss smell of grass. Grey skies, strong wind, rain in the air – another traditional English party had begun.

But not everyone there matched the stereotyped image of the modern soft-drug user. "I used to live round here until 20 years ago. I heard about the march and came along," said Charles Kell, 82, a retired electrician from Surrey. "It's really nice that these people have turned out. It's a bit of life. Nowadays, most British people can't even be bothered to turn out and vote, which is why it's so good to see these people here. It used to be like this round here when I was young. People used to talk to each other. Now it's dead."

Though he wouldn't admit to having actually smoked the weed himself, Mr Kell is adamant that full legalisation should happen soon. "Legalise it? Yes – but then the Government will probably do that one day. But of course they'd probably tax it, and how many of these people here would like it so much then?"

The complexities of future taxation policy didn't seem to be on the minds of most of the marchers. Spearheaded by the formidable elite shock troops of the University of East London's English folk heritage fanatics – resplendent in woodland green and papier-mâché oak and vine leaves – the march for dope began to shamble its way in the general direction of Brixton, some two miles away.

The inevitable slackening in pace and tendency to slow to a smiling standstill was countered by the pulsating rhythm of the drums of a London samba troupe. Without them, the march would possibly have faltered at the first zebra crossing. Half a mile or so along the Brixton Road the placards started to droop lazily. "Legalise today", "Get high tonight" and "Which Bush killed – George or ganja?" bobbed in and out of sight.

The throng rapidly grew from a few hundred to a respectable 2,000 – speeding up, slowing down, stopping to roll just another one or open another Bacardi Breezer, but always to the beat of downtown Rio. Beaming from behind an impressive four-incher, Ben, 24, a builder from Hertfordshire, was extra happy. "It's my birthday. This march happens every year on my birthday. Brilliant! My first spliff was at school when I was 13. No harm, no problem. Being a builder, I spend a lot of my days at the bottom of holes. Cannabis makes any boring work more bearable."

By now, much of Brixton knew that the parade had hit town. Many of the locals came out to watch in the only way that seemed appropriate. Mark, a Jamaican Brixtonian, plonked himself on a garden wall to watch the walk pass. Fumbling with a small-scale replica of the local narcotic speciality – the Camberwell carrot – he greeted the liberators with all the glee of a Parisian in 1944. "I've been smoking this stuff since I was 10. I'm 54 now. No health problem!" His friend Robert was equally effusive. "Spliff a day keeps the doctor away, man. I've smoked it since I was 12, no problems, and I'm 41 now. My girl used to have asthma – no problems now. If there is trouble when someone smokes it, it's because there was a problem in that person, not with the weed."

By 2pm Brixton shopping centre was near, but the wild men of the University of East London were only just now reaching cruising velocity. Dave, 25, an anthropology student, explained how hemp was part of old English tradition. Sidling dreamily past pawnbroker and job centre, he set out his vision for a return to pastoral, folksy Britannia. "It's been here for centuries. Queen Victoria used it to relieve period pains. It has all sorts of uses. It's crazy to ban something that is so useful."

The firmness of Dave's belief was only slightly compromised by the fact that on his head was an oak glade. Hey, nonny, no. A fellow Green Man – the mythological figure of fecund pleasure – tried to shed more light, but, his face and mouth obscured behind a giant paper ivy leaf, only a grey beard could be seen to move. For the record, he said something like: "Hmmnnffffnnn, uh?"

Past the heart of Brixton central the march now had a fair proportion of the youth of Saturday afternoon. Local entrepreneurs were not slow to latch on to the passing marketplace in front of them. "Weed, sensi, skunk, weed, sensi?" For Juanita Human, 20, a visitor from Johannesburg, it was the best thing that she had seen in Britain yet. "It's a great medicine. I prefer it to alcohol, which really is dangerous. But one thing: it's much cheaper in South Africa."

And then the climax: Brockwell Park. If this is a vision of how Britons might spend their leisure time in a few years' time, Britain is going to be a strange place indeed. The prototype may be pastoral Glastonbury, but the reality is a pleasure ground set out with stalls to cater for the new tastes, and some old needs – the Seedsman Cultured Cannabis, the Advisory Service for Squatters, Port Royal Jamaican Food, Free Radical Sounds, Chicken and Chips.

"Our message is: 'Don't flunk it, Blunkett'," said organiser Jerry Ham. "There's been some progress in the law, but we now worry that we're maybe moving into a grey area, and we don't want that."

But yesterday afternoon, the only grey in sight was swirling up into the skies.