At a time like this Sir Ian Blair can perhaps be excused his historical exaggeration. The hunt for the London suicide bombers, the Scotland Yard commissioner says, is the biggest challenge that the Metropolitan police have ever faced.I'm sorry, Sir Ian, but that's just not true. Bigger than Chartism? Bigger than the Fenians? Bigger than the unemployed demonstrations in the 1880s? Bigger than the Mosleyites in the 1930s? Bigger than the Blitz? Bigger than the urban riots of the 1980s? With respect, I think not.For large parts of the Met's history - perhaps indeed for most of it - Londoners and their police have not lived easily together. The police were imposed on London in the riotous 1820s, and the imposition defined the relationship for decades to come - until well into my lifetime. Policing with consent may be the buzz phrase today but it was not always so. For most of its history the Met tried to recruit from outside London - from the army and from Scotland in particular - in order to maintain a distance from, not a closeness to, the London community. The mythology summed up in the old Dixon of Dock Green image of the 1950s was very largely just that, mythology.That's not to say that there was no truth in the image of the local, avuncular bobby. But it was never the whole story. In Cold Bath Fields in 1833, just around the corner from where the Guardian's offices now stand, police attacked a political reform meeting without provocation and an officer, Constable Culley, was killed by the crowd. Local people made their views clear when a coroner's jury brought in the astonishing verdict of "justifiable homicide". Not much community policing there.And so it continued. Half a century later, when police trampled a bystander to death in the original Bloody Sunday riots of 1887, the funeral generated one of the largest and angriest crowds of the 19th century. As the coffin of Alfred Linnell - decorated with the words Killed in Trafalgar Square - was carried to Bow cemetery, the police were greeted with repeated chants of: "That's your work!" Shades of Blair Peach a century later.What the Metropolitan police need, Margaret Thatcher once said in that peremptory way of hers, is "support not criticism". Yet this was never a universal view. Most people accepted the police, then as now. But support had to be won, and too often it was not. Not for nothing did one young Londoner, Alfred Hitchcock, sum up his views of the police in ways many shared. "I'm not against the police," the great film-maker once declared. "I'm just afraid of them."It is hardly surprising that those kinds of views are being expressed yet again this week, after the Met shot Jean Charles de Menezes dead on a tube train last Friday. In the aftermath of this month's bombings there may be as many as 3,000 armed police on the capital's streets, a number that far exceeds either the Met's specialist armed response units or the armed police who guard key public figures and important buildings. In current circumstances that's an awful lot of people in our midst who are all a hair's breadth away from firing at a suspect. With the revelation of the "shoot to kill to protect" policy, suspicions must be expected and questions answered. Like Hitchcock, reasonable people may be afraid. The Commons home affairs committee ought to sit immediately.Shootings by the police still remain incredibly rare in this country. In most places, years can go by between one police bullet being fired and the next. But the key question is when it is legitimate. From what has been reported, the officer who killed the young Brazilian last week appeared to act in line with the rules issued under Lord Stevens. In extreme circumstances, he said, an armed officer can shoot a suspect in the head if the intelligence suggests that he is a suicide bomber who poses an imminent danger to the public or the police.At first sight this could be depicted as a radical extension of police guidelines. But is it? A closer examination reveals that it is more a particularisation of appropriate rules that have governed firearm use by police and the military in Britain, especially Northern Ireland, over many years. These rules contain a number of basic principles: that any force used should be the minimum necessary, that force should be used against particular targets rather than indiscriminately, that no more rounds should be fired than is necessary, that an audible warning must be given and that the officer should be certain that the suspect is about to take offensive action.What is important, though, is that the guidelines do not have the full force of the law unless and until they are tested in court. Now, tragically, we have such a case and the issues must be put to the test. Part of one's heart sinks. Matters of this kind have been a legal battleground in Northern Ireland for decades, not least in the Saville inquiry, mainly because of republicans' determination to depict themselves as the victims of British state violence but also because there are real issues of liberty and law at stake. Even Northern Ireland's often-maligned courts have found, for instance, that the mere failure of a suspect to halt when challenged does not justify shooting him.We need to be prepared to make a similar distinction in the De Menezes case. Just because some mischief makers are already trying to imply that the Met has a shoot-to-kill policy against young, dark-skinned men - which it doesn't - this does not absolve either the Met in general or the officers involved from having to answer for their decisions before a proper tribunal. An innocent man has been killed in the effort to protect other innocent people. That action must be judged. That would happen in Iraq and Israel - and in the British army. It should happen here too.The police should not panic about this. Over the past 25 years, largely since the Scarman report, there has been more reform of London's police than in the preceding century and a half. A police force has become a police service. The quality of leadership has been transformed - compare Ian Blair with David McNee - as have the quality and diversity of its officers. Much remains undone. There are too many boys with toys. Politicians still veer away from constructive criticism of the police.But this should not blind us to what has changed. The relationship between the police and Londoners is unrecognisable from what it was. In the McNee era there would have been no apologies and the force would have drawn tightly together, maligning the critics rather than facing up to the criticism. In the Blair era that is not necessary. Dreadful errors will still happen, from which lessons must be learned. The police are not magicians. But the public has learned to trust the police, errors and all, and I think they are right to do so.