John Harris: What do we want? History! When do we want it? Now!

The Heretic: What sort of protest group is it that includes Tory leadership candidates, the Liverpool team, Rupert Murdoch and Jeremy Clarkson?

On close inspection, however, their excitement seemed a little misplaced. In big, bold capitals, the tickets read, "Live 8 Live On Screen", a hucksterish description of the bolted-on enterprise whereby second-class gig-goers would be admitted to Hyde Park to watch the show on giant televisions. Given that the concert would be beamed to most TVs on the planet, the idea seemed rather bizarre - though no one in the queues seemed to think so. According to press reports, there was much talk of "the cause", plenty of enthusiasm for the idea of "taking part", and even more chatter about the epochal nature of the day. "This is going to be the biggest event of our generation," said a 19-year-old student named Ben Archer.

In tandem with Live 8's location, it all put me in mind of that watershed event at the tail-end of summer 1997, when thousands watched an event of allegedly similar importance on those self-same giant screens. Indeed, it doesn't seem entirely misplaced to see the funeral of Diana, Princess of Wales, far more than the original Live Aid, as the decisive harbinger of a very modern archetype: those periodic spectacles that suspend the usual rules of our atomised society, and are built around huge, emotion-strewn crowds. The big draw, it seems, is the thrilling feeling of significance. Poverty might have been the focus for Live 8's organisers, but another concern seemed to be playing on the minds of its audience. Just before Coldplay's set came the inevitable announcement: : "You're part of history."

For the generations who have experienced slightly more tumultuous times, all this must seem very strange. My grandparents, for example, had no need to make a point of participating in history - their involvement was obvious, in the contemporaries they lost to war, the bombers that flew over their homes, and the industrial strife that marked their working lives. Now, however, we seem to think we can connect with big narratives only via coach trips to big events, and the magical invocation of the "H" word. (I don't know whether the Make Poverty History campaign thought of all this when they came up with the slogan, but it's surely a beautifully subliminal aspect of it.)

None of this is, of course, intended to besmirch the motives of either MPH's founders, or most of the musicians who took part in Live 8. It should, however, give us pause for thought when it comes to the political momentum that the event and the wider campaign can generate. Specifically, it raises doubts about Bob Geldof's idea that Tony Blair - with whom, it has to be said, he seems to be getting absurdly friendly - might somehow seize on the energy created yesterday and stride into the G8 summit claiming "the largest democratic mandate ever collected in the history [that word again] of this planet". For a start, that notion seems to embody a view of the British government, seen also in Richard Curtis's icky G8 drama The Girl in the Café and Bill Gates's on-stage tribute to Blair and Brown, which sacrifices realpolitik to something bordering on whimsy. More important, however, it ignores what MPH's inflation to such huge, consensual, supposedly historical proportions actually means - because once the Diana syndrome kicks in, the first casualty is meaning.

All this may sound like so much hard-boiled leftist cynicism (indeed, those who are churlish about Live 8, in the elegant words of Coldplay's Chris Martin, stand accused of being "knobheads"). By way of a defence, I am more than happy to identify the same syndrome in at least one event that, for people with my kind of politics, has long been talismanic. When we marched against an attack on Iraq in February 2003, many of us were buoyed by the idea that we were witnessing the birth of some unstoppable movement. At each successive anti-war demo, however, the numbers plunged. Here was Diana-ised protest: that all-important fix of righteous togetherness, the sense that history was being made, and no desire for a sequel. Indeed, if one of the march's more prevalent slogans was taken at face value, there was no point. "Not in my name," read all those placards. Once that had been established, wasn't the job done?

You could sense the same kind of feeling among the Live 8 crowd. "It's not just about the music," one of the screen-watchers assured a reporter. "It's about the cause. I went to Live Aid because I believed in what that was about. Now I believe in this." Unfortunately, her crystallisation of the concert's grand intentions did not include any reference to MPH's stated aims. Live 8, she said, amounted to "a way of people showing that they care". Objectives are thus pushed into the background: as the cliché would have it, it's the taking part that counts.

According to the Daily Mirror, six million Britons have those white wristbands. As well as the audience and performers at Hyde Park, the campaign's cheerleaders include almost all the Tory leadership candidates, the cup-winning Liverpool team, Jeremy Clarkson and Rupert Murdoch. Last Thursday, one tabloid newspaper bowed to the inevitable and implored Princes William and Harry to show up at Live 8 and lend the event the support of the monarchy (shades, once again, of the strange week following Diana's death). In that context, it hardly seems churlish to wonder exactly what the supposed mass consensus on MPH's aims represents. What balance of emphasis on aid, debt and trade are we all after? Might the fact that the last issue has been so sidelined, despite its key to any long-term relief of poverty, prove that this is the kind of gargantuan coalition whose size renders it vapid? To put it more dramatically, did yesterday represent the point at which the Global Development Movement fell into a very moronic inferno?

Perhaps the most crucial point is this. I'm not sure you can hold the leaders of the industrialised world to account on the basis of fuzzy sentiment. Indeed, the Blair experience rather suggests that by whipping it up, you may well hand them a gift. Think about it this way: mere months before the Diana crowds massed in Hyde Park, the myth that has sustained Mr Blair in office was born when he was swept into Downing Street on a wave of similarly open-ended public feeling. Ever since, both left and right have been trying to fathom out exactly what Britain was thinking. Handily for Mr Blair, his victory could actually mean whatever he fancied.

Thus, come the crucial Gleanagles press conference, the Prime Minister may well cast his eyes over an agreement of precious little significance - and note: G8 communiqués have been making hand-wringing references to the problems of Africa since the mid-1970s - and then blithely announce that Clarkson, the Liverpool team and the 55,000 people who sat by the screens have all got pretty much what they wanted. And then, with no little fatalism, we will nod, and think for a minute, and wonder when the next Diana-ish spectacle might arrive.

John Harris is the author of 'So who do we vote for now?'