The principal cause of man's unhappiness is that he has learnt to stay quietly in his own room. If our needs are not met, if justice is not done, it is because we are not prepared to leave our homes and agitate for change. Blaise Pascal ("the sole cause of man's unhappiness is that he does not know how to stay quietly in his own room") couldn't have been more wrong.

We do not starve, we are not arbitrarily imprisoned, we may vote, travel and read and write what we wish only because of the political activism of previous generations. Almost all MPs will acknowledge this. Were it not for public protest they wouldn't be MPs.

Yet, though the people of this country remain as mild and as peaceful as they have ever been, our MPs have introduced a wider range of repressive measures than at any time since the second world war. A long list of laws – the 1997 Protection from Harassment Act, Terrorism Act 2000, Regulation of Investigatory Powers Act 2000, the 2005 Serious Crime and Police Act and many others – treat peaceful protesters as if they are stalkers, vandals, thugs and terrorists. Thousands of harmless, public-spirited people now possess criminal records. This legislation has been enforced by policing which becomes more aggressive and intrusive by the month. The police attacks on the G20 protests (which are about to be challenged by a judicial review launched by Climate Camp) are just the latest expression of this rising state violence. Why is it happening?

Before I try to answer this, let me give you an idea of just how weird policing in Britain has become. A few weeks ago, like everyone in mid-Wales, I received a local policing summary from the Dyfed-Powys force. It contained a ­section headed Terrorism and Domestic Extremism. "Work undertaken is not solely focused on the threat from ­international terrorists. Attention has also been paid to the potential threat that domestic extremists and campaigners can pose." I lodged a freedom of information request to try to discover what this meant. What threat do ­campaigners pose?

I've just been told by the police that they don't intend to reply within the statutory period, or to tell me when they will. I'll complain of course, and (in 2019 or so) I'll let you know the result. But Paul Mobbs of the Free Range Network has found what appears to be an explanation. Under the heading "Protect[ing] the country from both terrorism and domestic extremism", the Dyfed-Powys Police website repeats the line about domestic extremists and campaigners. "In this context, the Force was praised for its management of the slaughter of what was felt to be a sacred animal from the Skanda Vale religious community in Carmarthenshire." You might remember it: this Hindu community tried to prevent Shambo the bull from being culled by the government after he tested positive for TB. His defenders sought a judicial review and launched a petition. When that failed, they sang and prayed. That's all.

Mobbs has also found a bulletin circulated among Welsh forces at the end of last year, identifying the "new challenges and changes" the police now face. Under "Environmental" just two are listed: congestion charging and "eco-terrorism". Eco-terrorism is a charge repeatedly levelled against the environment movement, mostly by fossil fuel lobbyists. But, as far as I can discover, there has not been a single recorded instance of a planned attempt to harm people in the cause of environmental protection in the UK over the past 30 years or more. So what do the police mean by eco-terrorism? It appears to refer to any environmental action more radical than writing letters to your MP.

The Association of Chief Police Officers (Acpo) now runs three units whose purpose is to tackle another phenomenon it has never defined: domestic extremism. These are the National Extremism Tactical Coordination Unit (Netcu), the Welsh Extremism and Counter-Terrorism Unit and the National Public Order Intelligence Unit. Because Acpo is not a public body but a private limited company, the three bodies are exempt from freedom of information laws and other kinds of public accountability, even though they are funded by the Home Office and deploy police officers from regional forces. So it's hard to work out exactly what they do, apart from libelling peaceful protesters. I wrote a column in December about the smears published by Netcu, which described villagers in Oxfordshire peacefully seeking to prevent a power company from filling their local lake with fly ash as a "domestic extremist campaign". It also sought to smear peace campaigners, Greenpeace and Climate Camp with the same charge. Netcu's site went down on the day my column was published and hasn't been restored since. But we have only patchy evidence of what else these three unaccountable bodies have been up to.

They appear to have adopted the role once filled by Special Branch's counter-subversion campaign, which spied on Labour activists, including Jack Straw and Peter Mandelson (sadly the spooks failed to bump them off while there was still time). But as Paul Mobbs points out in his new report on Britain's secretive police forces, today the police appear to be motivated not by party political bias but by hostility towards all views which do not reflect the official consensus.

Mobbs proposes that mainstream politics in Britain cannot respond to realities such as global and national inequality, economic collapse, resource depletion and climate change. Any politics that does not endorse the liberal economic consensus, which challenges the concentration of wealth or power, or which doesn't accept that growth and consumerism can be sustained indefinitely, is off-limits. Just as the suffragettes were repressed because their ideas – not their actions – presented a threat to the state, the government and the police must suppress a new set of dangerous truths. By treating protesters as domestic extremists, the state marginalises their concerns: if people are extremists, their views must be extreme. Repression, in a nominal democracy, cannot operate accountably, so the state uses police units which are exempt from public scrutiny.

I am sure Mobbs is right. There is no place for dissenting views in mainstream politics. I was told recently by a Labour backbencher – a respected MP untainted by the expenses scandal – that "if the door was open just an inch to new ideas, I would stay on. But it has been slammed shut, so I'm resigning at the next election." Our grossly unfair electoral system, which responds to the concerns of just a few thousand floating voters and shuts out the minor parties; the vicious crackdown on dissent within parliament by whips and spin doctors; the neoliberalism forced upon governments by corporate power and the Washington consensus; the terror of the tabloid press – all combine to create a political culture which cannot respond to altered realities without collapsing. What cannot be accommodated must be suppressed.

The police respond as all police forces do; protecting the incasts from the outcasts, keeping the barbarians from the gate. The philosophy of policing has not changed; they just become more violent as the citadel collapses.

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