It seems to me that there might be grounds for a complaint to the Independent Police Complaints Commission ( http://www.ipcc.gov.uk/
) that the police breached the human rights of those in the autonomous bloc. In previous years, police targeting of anarchist groups could be justified because Mayday events were not "legal". However, this year, because the bloc took part in the official march, the police actions from the beginning of the march could be argued to have been harassment exclusively on the grounds of political opinion (because it was the anarchist part of the march).
Thus, it might be argued that they breached both Article 2 of the Universal Declaration of Human Rights: "Everyone is entitled to all the rights and freedoms set forth in this Declaration, without distinction of any kind, such as race, colour, sex, language, religion, political or other opinion, national or social origin, property, birth or other status." ( http://www.un.org/Overview/rights.html
) and also Article 2.1 of the International Covenant on Civil and Political Rights: "Each State Party to the present Covenant undertakes to respect and to ensure to all individuals within its territory and subject to its jurisdiction the rights recognized in the present Covenant, without distinction of any kind, such as race, colour, sex, language, religion, political or other opinion, national or social origin, property, birth or other status." ( http://www.hrweb.org/legal/cpr.html
), which the UK has ratified.
The sound system is beside the point, the possible harassment of the bloc was of all members, not just those in charge of the system, and was from the very start of the march when the police apparently showed no interest in shutting it down. The simple fact of the concentration of police presence exclusively around those waving red and black flags could possibly be described as harassment.
Just something to think about (I was just ahead of the Autonomous Bloc in the march).