I've never been too drawn by flashmobs. If people chose to gather clandestinely and suddenly burst out into song en masse or into a dance routine, it seems harmless enough. Maybe a burst of a surprise activity in a public place is a good thing, momentarily shaking people's assumptions of what is normal, and maybe raising their spirits.

But the idea of a flashmob of people meditating? In Trafalgar Square? I didn't care for the idea. It seemed to either present meditation as a display of the weird and wonderful, or be making an ostentatiously pious comment about the dehumanisation of urban contemporary life. Weirdness or smugness, why go for it?

I have a history of unease about meditating in public places. It has never seemed to be putting out clear or useful messages. When I lived in New Mexico, a group was organised to meditate near the base at Los Alamos where the atomic bomb had been developed. It felt as if we were sitting there as an overt display of an alternative: the good outside the gates and the bad inside. But what did we achieve? Whom did we convince, or whom did we sway? Coasting along on a cloud of conscious virtue is often too easy a ride.

At Los Alamos there was, it must be admitted, a sense of gravitas. Ambling in a long line after Vietnamese Buddhist monk Thich Nhat Hanh felt plain silly. "Tai" had given a talk in Caxton Hall, with his usual gentleness and luminosity. He then proposed we all follow him out of the doors in a slow walking meditation to St James's Park. It was the end of a warm summer's day, as I recall, and office workers were spilling outside out onto the pavements outside crowded pubs. We walked past them eyes, downcast, past pubs, over zebras, along busy roads, a long slow column of folk. The contrast between the loudness of the drinkers and the soberness of the walkers gave the impression of a criticism that was surely not intended by Hanh.

So what to do about the suggestion that came into my email inbox to go and meditate as part of a flash mob in Trafalgar Square? It could be exhibitionism, or – at worst – a glib use of what is a profound and extraordinary discipline. There was an awful lot against it. And yet, dear reader, I went.

I went because I wasn't sure. I could see that I had been embarrassed in Los Alamos traipsing after Tai. Part of me had been uneasy about breaching conventions and had felt vulnerable and exposed. But if I had felt undefended in a public space, then surely, I thought, I ought to explore that.

And come down to it, it is true that I do believe without a smidgeon of doubt that meditation is an unmitigated good. So why be shy about physically standing up (or rather, sitting down) for it?

I went along expecting unease, and expecting challenge. I was wrong on both counts.

Trafalgar Square, 10 minutes before the secret lift-off, was full, as usual, with summer crowds and precious few people that I could clock as potential meditators. It was warm and I sat on a bench to wait.

I didn't hear the lone woman singing at 6.30pm who was the signal, but I saw something even more startling.

Across the square, from all sides, apparently uninvolved idlers strode forward purposefully to the designated area between the two large fountains. It was like Superman suddenly emerging from unobtrusive Clark Kent – or rather, around a hundred or so Supermen.

It was simple to be impelled along on that sudden wave and to just sit down among the throng. And a deep silence immediately arose. I was astonished. There was a sense of naturalness and openness. The steady sound of the water in the two fountains, the grumble of traffic masked behind them, and a light hither and thither breeze. It felt not so much a comment on mad commercial London but more a coexistence with it – unifying in its effect rather than polarising.

After 20 minutes, the low growl of chanting "Om" began and it rolled back and forth in waves, rising and falling until 7pm struck. And then it was all over. Standing up, the world felt different. I would have liked to have made eye contact with someone, or even hugged someone. But being British, I reverted to the conventions of public space. I picked up my things and left to catch my bus.

I don't know if the exercise showed anybody anything or made any point at all. But I do know that for me it was a half an hour of purest sanity.