Rob was a predictably tedious anarchist, with tediously predictable views. Standing beneath a hanged effigy of Sir Fred Goodwin, he explained how the police work. “They aren’t here for us, they’re here for the bankers,” he told me. “But mainly, they are here for a fight.”

Having just watched protesters smash up the Royal Bank of Scotland, I was not in the mood. Not only was Rob wrong, I told him, he was the worst kind of wrong — a clichéd wrong. Seven hours later — seven hours of detention without food or water — I had come to believe that I was the one who had been naive.

Wednesday’s police operation against G20 protesters was, by most accounts, a success. Minimal violence, stoical police, and London back up and running a few hours later. But there is another story. It is the story of how a largely peaceful protest that happily coexisted with a Starbucks and an HSBC — each just one brick arc from anti-capitalist destruction — eventually turned violent. Most of all, it is the story of how the police wilfully criminalised and alienated 4,000 innocent people.

The police tactics were simple. At the first hint of trouble, they enacted a long-planned strategy — trapping and detaining all the protesters, violent or not, outside the Bank of England. Tom Brake, a Liberal Democrat MP there as an observer, spoke to an elderly couple unrelated to the protests who were nevertheless caught in the cordon. He said that they feared for their safety. Later he would tell me he feared for his own safety.

Once established, the cordon slowly squeezed — each police charge rolling past any protesters who refused to move, battering them. No one was released. If I were to design a system to provoke and alienate, I could not do better.

It is difficult to say when though, for me, the phoney war of Threadneedle Street became the battle of Threadneedle Street. Perhaps it was when four drunk thugs, gleefully filmed by every news channel, started taunting police. Perhaps it was when Jack Bright, a slight, softly-spoken twentysomething who had been sitting during a police charge, struggled out of the crowd with blood dripping from his forehead to his chin. Perhaps it was when the first hooded idiot threw a glass bottle, or perhaps it was when an enraged crowd, by then detained without charge for five hours, began chanting: “Let us out.”

What is certain is that by the time I saw a scared, disoriented young woman flee one police charge, only to be knocked down and hit by another, relations between the crowd and the Met were broken.

It is not as if, at the start, the police were short of friends. A DJ giving a performance from the steps of the Bank of England interrupted his reggae music to tell the crowd to let the police do their job. When some protesters threw bottles, others handed potential missiles to the police line.

Did the police want violence? Perhaps not as an institution, but if the few-rotten-apples argument can be applied to the protesters, it can be equally applied to the police. Pushing back the crowd, one officer told me that he needed neither provocation nor justification to hit someone with a truncheon. A Times colleague, on the other side of the barricades, saw two riot police putting on their helmets. One said to the other: “Now the fun begins.”

At 8.45pm, as the last protesters were allowed to leave, I asked the police about the tactics. One said the intention was to keep demonstrators there until all they wanted to do was head quietly home. That does not explain the slow territory squeeze. Another suggested that keeping the crowd until it got violent was a useful way to identify troublemakers before the G20 proper began.

Those are good operational strategies. But they are not a justification. The police should not have the authority to enact collective punishment. Just as the people who attacked the officers were idiots, so the actions of the police made that violence inevitable.

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