News that police have stopped a plan to set off explosives during the G20 summit this week is going to make everyone concerned even more twitchy — and frankly I wouldn't have thought that was possible. Police have been warning for weeks that serious disruption is planned. Protesters have been warning for weeks that police are coming down too heavily. Some protesters are trying to disown other protesters. Nerves are fraying everywhere you look in the countdown to the demonstrations planned in London on 1 April.

I went along to the Put People First march on Saturday to see if I could get any feeling for who might be around this Wednesday. It was interesting, with a better turnout than I had feared, and a cheery mix of marchers, from woolly-jumper-wearing communists through militant unionists to rainbow-tie-dye trousered hippies. In a funny way it was 1978 all over again.

But the centre of attention was the anarchists. Sorry, I'll try that again in true pantomime style: the ANARCHISTS (boo, hiss!!!). In the middle of their line-up was a fairly small — 100-200 at a guess — group of young people in black hoodies, black baggy trousers and, in some cases, a bandana across their faces. Some of them waved red and black anarchist flags. And across the front of the bloc they carried a large banner promising direct action, and suggesting that the bosses and bankers should be "first against the wall" (a sentiment I believe many across the nation would vehemently support).

But it wasn't really their natty dress-sense or their flags that made them stand out so distinctively from the rest of the march. It was the very special policing tactics that were focused on them: the anarchists, the police seemed to feel, were such an imminent danger to society that they needed to be 'kettled' — in other words, to have three police vans crawling along blocking their left-hand side, and a tight line of police one behind another on their right-hand-side, to make sure there was no possibility of break-out.

I've seen 'kettling' done even more intensively than this, at a recent Smash EDO protest in Brighton, for example, where the police were lined along the front and back of the group as well as down the sides, and I (walking my kid home from school) was warned to get out of the way as if it was an advancing army, rather than just 50-60 protesters. This small group in the 'kettle' was as resentful as you would expect, and a horrendous mood hung over the small event. There, as here at the Put People First march, with the cops wearing those nice bright yellow jackets, it was as if someone had taken a giant highlighter pen and drawn a line around the anarchists to indicate "Here be dragons".

You may wonder, who are these young anarchist folk, and why are they attracting so much attention? Over the last few years the movement in Britain has really put down roots, setting up social centres, book fairs, and newsletters around the country. A basic tenet of the culture is anti-police, and this is often fed by local long-running protests – anti-arms, animal rights, anti-road or pipeline – which sour relations with the local coppers, who almost inevitably handle the protesters as if they are radioactive. Local information will have been fed through to the Metropolitan police, identifying this particular bunch as trouble-makers (most of them probably have their own files – hence, they say, the bandanas across their faces). And so, we have the kettle.

Let's not be naïve here and say – as protesters far too often do – that it's all the police's fault, they were provoking us. The anarchists want action, they are delighted that the system is imploding on itself, they may well wade joyfully into battle with the police on Wednesday. And if police are dealing with intelligence of explosive devices, you can imagine that they're going to be feeling less than amiable towards the activists. But similarly, if the police try to claim that it was all the fault of the protesters, caution will be needed.

It could all get very messy indeed, and extremely careful monitoring is needed of police behaviour at the G20 protests (Climate Camp included). I must admit I feel pricklings of fear about it all, and so do many others. Because if I was a young person, fed up, no job on the horizon, watching my future implode thanks to a system I did not believe in, and I was being treated as a criminal by the police for simply wearing black, I too might feel stirring resentment. Is this really the best way to handle protesters?