I have no idea if this is really written by Alex Callinicos. Its a good enough argument to be from him. I found it online here;

 http://info.interactivist.net/comments.pl?sid=3628&cid=1703

"The Truth about Sunday"
Alex Callinicos

It is a matter for great concern that arrests took place during Sunday's demonstration, and the British organizers of the ESF have been working to secure the release of those arrested. We need to make sure that everyone is freed. Nevertheless, there are many inaccuracies in Enric's statement.

In particular, it is quite untrue that the police were protecting the stage in Trafalgar Square on Sunday. The demonstration was, at the request of the UK Coordinating Committee, organized and stewarded by the Stop the War Coalition and its partners in the British peace movement, the Campaign for Nuclear Disarmament and the Muslim Association of Britain. The stewards also provided a cordon to protect the stage.

It is very fortunate that they did. Had they not been there I have no doubt that the rally and concert would have destroyed by the same Black Bloc that had — shamefully — disrupted the anti-fascism plenary the night before, assaulted the black co-facilitator and stolen his mobile phone.

I saw the assault on the stage on Sunday. What happened was that the Black Bloc tried to break through the line of stewards protecting the stage. This involved attacking the stewards — mainly women and young people, many of them people of colour.

The pretext for this assault was that there had been arrests earlier on in the march. Why attacking the rally was the appropriate response to these arrests is beyond me, since it was the police who had made the arrests, not, for example, Aleida Guevara, who was speaking from the stage when the attack took place. The demonstration organizers had been working with Italian comrades to secure the release of those arrested, and an announcement about the arrests was made from the stage.

Of course, the violence that had been introduced into the demonstration by the Black Bloc encouraged the police to intervene. They entered the area around the stage and started filming the stewards and the Black Bloc who were wrestling together. It was thanks to the intervention of representatives of the Stop the War Coalition and the National Assembly against Racism that the police were persuaded to stop filming and to clear the area.

During these few minutes I saw one individual being dragged off by the police. This was a horrible sight, but the arrest would not have happened if the Black Bloc hadn't attacked the stage. The police seized on the incident to start behaving aggressively everywhere, and a number of arrests took place (one Globalise Resistance activist was stopped under the Terrorism Act and arrested).

Fortunately this incident, like the one the night before, were minor blemishes on a hugely successful European Social Forum in London. A dose of London rain cooled things down in Trafalgar Square and the concert brought the Forum to a happy end.

Nevertheless, those who, like the Babels coordinators, supported the disruption of the anti-fascism plenary on Saturday night, or who said that 'nothing happened' there should reflect on the logic of their position. Sunday showed, once again, that those who bring violence into the movement draw in the violence of the state against the movement.

Of course there is room for debate for the organization and content of the London ESF — as there is of all the Social Forums. But I have no doubt that the perpetrators of these incidents are politically hostile to the entire Social Forum project. For example, the website of the Wombles, who were certainly involved in the attack on the Saturday plenary, describes the WSF and ESF as 'institutions which parallel the development of capitalist institutions of global governance' and as 'potentially dangerous' because they don't 'really challenge' capitalism. For people with such views, the claim that their actions were in defence of the WSF Charter of Principles is the purest hypocrisy.

Despite the efforts of their perpetrators, these disruptions did not spoil the London ESF. But we need to draw the lesson: violence doesn't belong within our movement, and neither do those who bring it into the movement.