FOR those of you not watching the television coverage of today's march against spending cuts in central London, let me offer you a vignette. On a big stage in front of the bulk of the protestors in Hyde Park, Brendan Barber, the head of the Trades Union Congress (TUC) warms up for Ed Miliband, the Labour leader, by screaming into the microphone about the injustice of the government's austerity mission. Meanwhile, Harriet Harman, Labour's deputy, stands to his side while Mr Miliband waits at the back. So far, so bad. Viewers who worry that the current Labour leadership is beholden to the unions and public-sector vested interests will hardly be changing their minds.

Then the public-relations nightmare really begins. At the exact moment that Mr Miliband arrives at the lectern and begins speaking, black-clad protestors who had broken away from the main march begin misbehaving in the shopping areas of Regent Street and Oxford Street. Topshop, the high-street retailer accused of tax-dodging by many of the protestors, comes under assault. Various other shops close down as a precaution. The Metropolitan Police have just tweeted that some of their officers have been attacked by lightbulbs filled with ammonia.

The television coverage shows footage of all this happening while the disembodied voice of Mr Miliband, speaking in Hyde Park, compares the march to the Suffragette cause, the civil rights movement in America and the anti-apartheid protests in South Africa. As the words "David Cameron, this is the big society!" tauntingly leave his lips, viewers are treated to the spectacle of police officers being charged at by protestors. Mr Miliband finishes his speech by invoking Martin Luther King.

This embarrassing juxataposition of the speech with the violence is, according to some angry left-wing bloggers and tweeters, the dastardly work of Sky News. They are mistaken. I was watching the coverage on BBC News, who were carrying the same images and the same speech. How could they not?

Now, of course, Mr Miliband was unable to see what was going on a mile of two away from Hyde Park. And it was a pure accident of timing that one breakaway march (which amounted to a tiny fraction of the hundreds of thousands peacefully demonstrating) turned ugly just as he began speaking.

But it was his choice to address the rally in the first place, the latest stage in his curious political strategy of appealing to people who are already certain to vote Labour. It was his choice to go for the moral bombast of the apartheid and civil rights allusions in his speech, which were hostages to fortune. And he must have known that some kind of violence would take place today: it is only three months since the smashing-up of Parliament Square by protestors opposed to higher university tuition fees. Those few seconds of news coverage will have reached many viewers (it is Saturday, after all) and those who did not see it will probably have it brought to their attention in the coming weeks by the Tories' attack operation, who must not be able to believe their luck.

It is not just the violent fringe of these protests that are a threat to Labour's credibility and electability, though. As Bagehot blogged yesterday, and as Philip Collins wrote in The Times, the decent and good-natured majority of the anti-cuts movement are often wilfully oblivious to the need for any spending cuts at all, and "the alternative" they campaign for is the old standby of getting assorted tycoons, banks and corporations to pay more in tax. Labour's decision to associate itself so closely with these groups (who, in their soft-headedness, are worryingly evocative of some of the company Labour kept in their 1980s dog-days) is, quietly but surely, damaging the brand of a party that is already seen by voters as too left-wing. It may also make Labour look swivel-eyed and perpetually angry, in the way the Tories once did. In the short term, backing the protests might help rattle the government. In the long term, which slice of the electorate is Mr Miliband hoping to win over that he cannot already count on?

As I watch the television coverage of the march making its way through London, and of an HSBC branch being smashed up by anarchists, my eye is drawn less and less to the angry protestors and more to the ordinary people milling around looking perturbed. Some are foreign tourists. The rest are called voters.