As slogans go, "Hands Off Nepal" has a lot to recommend it: it's simple, direct - and yet at the same time, touched with a certain exotic romance.

"What's going on in Nepal?" I asked one of the two compact men carrying the banner on which the slogan was painted.

From behind the cloth stepped a good-looking young man in a long blue coat. "A people's resistance movement is battling the Nepalese government." What's that got to do with Iraq and President Bush, I asked him. The United States, he replied, was backing the government.

Oh, I said: "Are you from Nepal?" He replied that he was from Turkey. What about him, I asked, jerking my thumb at one of the banner holders. "He's from Turkey, too."

"Are there, in fact, any Nepalese here at all?" I asked him. "Not as yet, but" - he hastened to assure me - "we are expecting some Nepalese friends later."

The anti-Bush demonstration in Lincoln's Inn Fields was called for six o'clock, but at the appointed hour, journalists and camera crews substantially outnumbered protesters.

I joined a line of 14 journalists to interview a shapely woman dressed as a beauty contestant: "Miss Flaming Planet". Then, I was introduced to a man named Phil, the organiser of the event. He was wearing a woolly hat. I asked him how he kept an event like this on-message.

Before Phil could answer, a sharp-faced man directly behind him intervened." Why do you ask that?" he said suspiciously.

"Why do you think it's an improper question?"

The sharp-faced man was not diverted by my answer-a-question-with-a-question technique. He returned to his main point.

"OK," I said. "Isn't it the tragedy of the Left that it has historically allowed itself to be infiltrated and captured by tiny sects with extremist ideologies? Aren't you worried it could happen again?"

The sharp-faced man answered with a superior air. "When you have a mass movement like this, it's impossible for it to be captured by a small group."

I looked up and down the south side of the square. The "mass movement" extended barely half the length of the railing. I'd seen larger crowds at poetry readings.

And, in fact, the people looked very like the crowds you see at poetry readings: mostly white, shaggy but clean in appearance, polite and vaguely bookish in speech - the last inheritors of the landscape-loving English radicalism of William Blake, Thomas Paine and William Morris.

I thought of the tens of thousands of marchers I'd seen at the big anti-war demo in October 2002, chanting Islamic slogans from under their caps and hijabs.

They had not been bookish or polite. There was nothing woolly about them.

But they were young and fierce - and numerous. It is they, not these ageing men and women carefully tucking away their litter, who represent the future of the British Left - if, that is, a politics that pooh-poohs the crimes of Osama bin Laden and rallies to aid the last-ditch struggles of the Ba'ath party of Iraq can in any meaningful way be called "Left" at all.

The war on terror has glaringly exposed the moral contradictions of contemporary political radicalism: a politics that champions the rights of women and minorities, but only when those rights are threatened by white Europeans; a politics that celebrates creative non-violence at home but condones deadly extremism abroad; and, perhaps above all, a politics that traces its origins to the Enlightenment - and today raises its voice to protect militantly unenlightened terrorists from the justice dispensed by their victims.

A woman pressed into my hands a mimeographed sheet touting the merits of the Democratic presidential candidate Howard Dean.

"You do know," I said, "that Dean now says that American troops must stay in Iraq?"

"But of course," she said in a lovely French lilt. "Now that we - I mean you - are there, we must stay to clean up the mess. There is no choice."

Good point. But not as good a point as this one I heard from a young member of the Socialist Workers Party, standing underneath a clutch of red banners. I goaded him a little: "Wouldn't Trotsky describe your allies in this coalition as religious obscurantists? And isn't the history of the Middle East that religious loyalties count for a whole lot more than ideology?"

Mike (the name he gave) shrugged me off. "People in the Middle East are fighting because their own governments are repressing them. They come to feel that they have no alternative - and the mosque is always open.

"But I can't help thinking that it's just not very realistic that people are going to kill each other because they say my God is better than your God. Give people freedom and an opportunity for something better: that's what they really want."

I said: "You know, you sound exactly like Paul Wolfowitz." He flinched.

At this point, another of those sharp-faced monitors slipped into the conversation - this one a shortish young man with East Asian features and angry eyes. "Be careful what you say: it's the press. They'll distort your quotes."

"Here," I said. "I'll open up my computer and type the quote right in front of you. You can see it for yourself and correct anything I get wrong."

"You can quote the content but still distort the context."

I agree that context is everything, and the context of this week's events is that many thousands of British people intend to converge on central London to protest against the overthrow of one of the most cruel and murderous dictators of the 20th century - and to wave placards calling the American president who ordered the dictator's overthrow "the world's number one terrorist".

It's a deeply shameful context, and though I would not quite endorse the verdict of the taxi driver with the poppy stuck in his dashboard who dropped me off at the demos ("Not many of them traitors out tonight, I see"), he at least saw something that they, with all their apparently abundant education could not: that the two leaders they most scorn are the latest in the long line of Anglo-American statesmen whose willingness to use force to defeat evil secured them their right to make bloody fools of themselves in Lincoln's Inn Fields and through the streets of London to Grosvenor Square.