At 5.54 of this BBC footage, an 'anarchist' shows his pass to police and moves through the lines.

We are already seeing the first indications that plain clothes officers were moving between the violent protesters and the police at Saturday's demonstration. The police have infiltrated anarchist and revolutionary communist groups for decades.

One undercover officer, Mark Kennedy, has spoken at length about his work. Those he targeted have complained that he was inciting them to be more violent:

They allege that he also made visits to Dublin to help train protesters and encouraged other activists to attack the police. This raises further questions about his role as an undercover officer and backs up suggestions he acted as an agent provocateur. [from the Guardian, emphasis added]

The police don't simply infiltrate these groups to gather information about them, they seek to shape their dynamics. They encourage splits and they promote those whose agenda suits their own. They endeavour always to have the extra-parliamentary left they want, the kind of left that can be relied on to distract attention from issues of substance and matters of general concern.

And at the weekend to some extent they got what they wanted. On the BBC and in much of the press we saw the familiar narrative, of a peaceful demonstration being 'overshadowed' or even 'hijacked' but a minority of 'mindless yobs'. The newspapers could print stories about how the West End was terrorised, about the spectre of communism or anarchism. Journalists could accuse those who had broken windows and thrown paint bombs of distracting attention from the peaceful majority. They could make the point repeatedly and so minimise serious discussion of the march and its objectives. The coverage was almost entirely predictable. It was predictable because it was in important respects stage managed by the police.

(There was one innovation. The UK Uncut movement has been handily confused in some people's minds with the Black and Red groups - something that must have been high on the Metropolitan police's list of Things to Do.)

Those who were on the march will know that the images splashed all over the papers had little or nothing to do with their experience. The demonstration was not derailed. But they might want to ask their friends what they made of it all, based on the coverage in the newspapers and on television. My guess is that their perceptions won't seem anything like the event as it appeared at first hand.

And the reason for that is simple enough. The state seeks to manipulate the media in order to protect the status quo from serious challenge. The spectacle of violent disorder is part of its repertoire of control. And the established media are eager to be manipulated in this way. The narrative is, as I say, familiar. Everyone knows what is required of them. The danger that the weird unanimity of the political establishment might come into focus is averted once more, as 'moderates' bravely denounce 'extremists'.

If we want to do something about this, then we have to become more communicative. We need to start talking about our experiences and try to explain to others how far removed from reality media coverage can be. And we need to start the conversation about political economy that the country needs and that the political class is hellbent on avoiding.

Part of that conversation should touch on reform of the systems of communication on which we rely and which, as at the weekend, so regularly betray our trust. March 26th matters for many reasons. For one thing it reveals to those who were there the gap between reality and the news agenda. It is up to us now to explore that gap and to take steps to close it.

Dan Hind's The Return of the Public explores the political significance of the media industry and argues for its wholesale reform as a necessary step towards effectual democracy. It has been shortlisted for the 2011 Bristol Festival of Ideas 'Best Book of Ideas' Prize.

Hind blogs at The Return of the Public.