For most of the past century, Britain's secret state bugged, blacklisted and spied on leftists, trade unionists and peace campaigners, as well as Irish republicans and anyone else regarded as a "subversive" threat to the established order.

That was all supposed to have been brought to a halt in the wake of the end of the cold war in the early 1990s. MI5 now boasts it has ended its counter-subversion work altogether, having other jihadist fish to fry (it will have soon doubled its staffing and budget on the back of the 9/11 backlash).

Whether those claims should be taken at face value must be open to question. But it now turns out that other arms of the secret state have in any case been stepping up to the plate to fill the gap in the market.

The Association of Chief Police Officers (Acpo) insists that its confidential intelligence unit – reported last week to be now coordinating surveillance and infiltration of "domestic extremists", including anti-war protesters and strikers – is not in fact a new organisation, but has been part of its public order intelligence operations since 1999, liaising with MI5 and its 44 forces' special branch outfits across the country.

But yes, Acpo's spokesman tells me, it is in the business of targeting groups such as those involved in the recent Gaza war protests, trade unionists taking part in secondary industrial action and animal rights organisations – though only if they break the law or "seek to break the law".

Now, that qualification could be used to cover a very wide group of political and industrial activists indeed: including all those students who have been occupying university buildings since the new year in protest at Israel's carnage in the Palestinian territories; all those engineering construction workers who staged mass walkouts at refineries and power stations over the past couple of weeks; and all those who blocked streets – or threw their shoes at police – around the Israeli embassy in London at the height of the Gaza bombardment in January.

Add to that the fact that Acpo, and the government as a whole for that matter, bandies around the term "extremism" without being able to make even a face-saving stab at what it actually means – "there doesn't seem to be a single, commonly agreed definition", Acpo's spokesman concedes – and you have a recipe for a new lease of life for the harassment and criminalisation of legitimate dissent, protest and industrial action.

In case there were any doubt about the kind of thing this intelligence outfit is up to, a recent advertisement for its new boss specified that the unit would be specifically working with government departments, university authorities and private corporations to "remove the threat" of "public disorder that arises from domestic extremism" using "secret data" and "sensitive source material".

But since Acpo operates as a private company outside the Freedom of Information Act – and the budget and staffing of its confidential intelligence unit are, well, confidential – who's going to hold them to genuine account?

It is this kind of blurring of the distinction between political violence and non-violent protest that has seen catch-all anti-terrorist legislation routinely abused in recent years. That's exactly what seems to have happened over the weekend, when police arrested nine people on the M65 motorway near Preston allegedly on their way to join George Galloway's Viva Palestina aid convoy to Gaza.

Security sources said the arrests were in connection with a "potential threat of terrorism in the Middle East" — and it seems they didn't mean a renewed Israeli use of white phosphorus and heavy artillery shells against Palestinian civilians in the Gaza strip.

Six have already been released, but the operation instantly delivered a "Galloway aid convoy link to terror suspects" headline in yesterday's Mail on Sunday, casting a shadow over the 150-vehicle convoy, including 12 ambulances and a fire engine, which is intended to transport £1m worth of aid and highlight the humanitarian crisis in Gaza.

The crudely politicised timing of these arrests — "security sources" have been quoted as saying the three still being held had been under surveillance for two months – underlines how easy it is to play anti-democratic political games once the mantras of terrorism and national security have been invoked. But the net can be thrown far wider under the even more meaningless badge of "extremism".