Deborah Orr: The catalogue of incidents that tell the Met is out of control

The Metropolitan Police are chest-deep in trouble. When Ian Tomlinson died of a heart attack at the G20 protests a week ago, the official statement issued by the Met described officers being hampered by missile-throwers as they tried to administer medical help to a bystander caught up in an anarchic demonstration. The police confirmed that they had had no previous contact with Tomlinson.

Now, thanks to video footage handed to The Guardian by a New York fund manager, it has become indisputable that there was "previous contact". A masked policeman is seen as he appears to hit a passive, ambling Tomlinson from behind on his thigh with a baton. The technique is used to make people's legs buckle under them.

Tomlinson didn't fall though, and the officer is next seen rushing him from behind and very forcibly pushing him to the ground. A number of other officers, from the City of London police, stand by as this occurs, and a member of the public helps Tomlinson to his feet. Not long after this footage was shot, he collapsed.

Until yesterday, the City of London police had themselves been investigating the incident, under the management of the Independent Police Complaints Commission. It speaks volumes that even a visiting US businessman understood that he was better off handing his evidence to the media than to the police force whose criminal behaviour he had filmed.

Neither Tomlinson nor the cameraman was an active participant in the protest. Both were legitimately in the area, as indeed were the vast majority of the protesters. It has to be stated that from the start of the protest it was clear that a small minority of participants were there to make trouble. Dressed in black and masked, they were tolerated and even defended by other protesters. Very early in the day, I watched two protesters with megaphones barracking the police as they detained and questioned a masked man. Later, as a group of masked protesters ran at police cordons, it was apparent that the sympathy of many onlookers was with the masked aggressors.

I do not believe that legitimate peaceful protesters attend events in masks, and consider that the failure of protest organisers to condemn such behaviour is damaging to the credibility of peaceful protest. But masked police are a far greater threat to civil liberties than masked protesters. The reasons why troublemakers at protests should cover their faces are nastily obvious. That goes for the police as well as for "anarchists".

Of course protests are highly charged. No sensible person wants them to get out of hand, and it is the job of the police to make sure that does not happen. But the police are not the neutral actors in these highly ritualised dramas that they purport to be. They see the staging of protests primarily as confrontations that are directed against them and treat them as battles that have to be won.

The police have come to view protests as opportunities to express their own political beliefs, and advertise their own frustrations. Protesters often jeer that the police are state patsies, unquestioning in their defence of their masters. The police, in turn, appear to go out of their way to confirm that this is so.

Any small suggestion that the police are there to protect and manage citizens exercising their democratic right to question political processes they see as misguided or wrong, has been jettisoned. Collectively, the police see all protesters as the enemy, and believe that any person who becomes drawn into a protest, however casually or innocently, is fair game and gets what's coming to him.

This is not the view of a few bad apples in the force, although there are indeed extreme elements in the police who are every bit as keen on promoting violence for its own sake as some protesters are. Instead, that view comes from the top of the command structure. The police think nothing of penning all protesters into confined spaces for many hours, in what they say is a technique that controls agitating minorities, but what is actually a technique that condemns all present to collective punishment, which sometimes continues for many hours.

This technique, known colloquially as "kettling", was in operation from the earliest stages of last Wednesday's protests. Its main consequence is to promote frustration and resentment in ordinary people who have committed no crime, but simply want to make their opinions – opinions that millions of people to some extent share – known. The police, I'm sure, believe that it puts people off involving themselves in further protests. No doubt it has that effect on some. Others, however, are further radicalised and hardened in their prejudice against the police. The police themselves exhibit great prejudice, and part of their strategy is to legitimise that prejudice by portraying all protesters as dangerous people looking for trouble. That's why, when the news of Tomlinson's death broke, the Met was tempted to mention the throwing of a couple of plastic bottles as if this was a sustained and wild attack by a mob that knew no sense. This fantasy was accepted and colourfully reported by many news outlets.

Many people were subjected to unprovoked violence by the police on April 1. Poor Tomlinson's treatment would probably not even have been reported, had he not died so soon afterwards. Mostly, people don't report their injuries because it is clear that they will not be taken seriously by those who inflicted them.

It is appalling that a man had to die before the behaviour of the police on April 1 began to be subjected to scrutiny. Footage of riot police attacking peaceful protesters at the Bishopsgate Climate Camp last Wednesday evening, then detaining them all for five hours before attacking them again, should have raised alarm. There, the police covered their numbers when legal observers exhorted people to take note of the details of officers who had used unprovoked violence against them. Again, those actions suggest that the police know they are acting illegitimately and are determined to evade the consequences of their behaviour.

Yet what are those consequences? Recent months have seen a slew of disturbing cases that scream of police prejudice, incompetence or unaccountability. The most high-profile of these was the de Menezes inquest, in which it became grievously obvious that a Brazilian electrician was shot dead at Stockwell Tube station after a catalogue of errors in the wake of the 21 July attempted bombings. These errors, apparently, were nobody's fault, and nor were the desperate machinations of the Met as they played for time in the aftermath of the killing, rather than voluntarily releasing an honest assessment of their failures and a sincere apology.

These cases may seem quite different. But again and again – from the Rachel Nickell debacle to the Barry George fiasco, and in the identikit cases of rapists John Worboys and Kirk Reid – the foul-ups of the Met have one thing in common. The police go into a situation with their minds made up, their strategies already laid out, and their justifications rehearsed in advance. They never acknowledge their mistakes, but always protect the officers who make them. So they never, ever, learn anything. The amazing thing is that they keep on getting away with it.

This time, they must be bludgeoned themselves into understanding that they have to change.