One night last December, having already spent five hours trapped by the Metropolitan police in Parliament Square, I was imprisoned on Westminster Bridge along with 1,000 other mostly young protesters, in sub-zero temperatures, for more than two hours. We were held in such a tight space that some suffered respiratory problems and chest pains: the symptoms of severe crushing. This is kettling, and in its strategic brutality and unabashed doublethink, it is the perfect hallmark for the Cameron era.

In a landmark ruling, the high court ruled on Thursday that the Met's use of the tactic during 2009's G20 protests was illegal. Their wider use of kettling, common throughout this winter's student and anti-cuts protests, is currently being challenged at the European court of human rights. Despite the high court warning that it must only be used as a "last resort catering for situations about to descend into violence", the Met are unrepentant. "At the heart of this case," they responded, "lies a vital public order policing tactic that prevents disorder and protects the public." They will appeal against the high court ruling, and continue to use kettling "where necessary".

While the tactic's origin is German (from "Kessel"), kettling is very rarely used by police forces other than our own; it's such a British verb, somehow – a darkly comic inversion of the national obsession with the serenity to be found in a nice cup of tea. And for a newly politicised generation of young Brits, it has become a common, though brutal, reality. It felt "like I'd been in a car accident", said one female student who had been kettled on Westminster Bridge. Invoking Hillsborough, a doctor present that night observed it was miraculous no one was killed – from crowd panic and surges that could have easily led to people toppling over the waist-high walls of the bridge and into the Thames below. Another video from inside the kettle that day was described as "appalling" by the chair of the Met watchdog, the Metropolitan Police Authority.

Beyond the physical danger, kettling is collective punishment, in violation of the Geneva conventions: a response to the brick-throwing of a handful of protesters that affects the peaceful ones, too, no matter how old or young, how sick or well. More than that, it is de facto imprisonment without trial. It is also police brutality at its most devious – the strategic version of a baton to the head. You want to have your voice heard, to speak out about injustice? How about we smother your esprit de corps, shut you off from your fellow citizens, and raise your temperatures until you do break the law? The nomenclature used on the police side, meanwhile, exudes Wordsworthian calm: they call it "containment", an attempt to mop protesters' brows in the interests of everyone's "public safety". It is impeccable Orwellian newspeak. As David Lammy asked the home secretary, Theresa May (who was busy denying that a kettle had even existed) : "Is not the point of a kettle, that it brings things to the boil?"

It is often observed that kettling is designed to dissuade people from coming out to protest: if anything, it has the reverse effect on those who've experienced it. As protesters finally shuffled out of the Westminster Bridge kettle in single file, after seven hours imprisoned in freezing temperatures without food, water, toilets or freedom of movement, I saw several of them look the police in the eye – for that was all they could see, beneath a riot shield visor and a raised black snood – and say, some with humour, some with anger – but all with total defiance, "see you at the next one, mate".

Freshly radicalised by these experiences, it is little surprise that on 26 March, so many young people chose to reject the police-approved TUC march and masked up, seeking freedom and solidarity in the anonymity of the black bloc. I say this to the police: why should protesters engage on your terms, when these are your terms?