“There were simply far too few police deployed on to our streets,” Mr. Cameron said of the initial days of the riots this week, “and the tactics they were using weren’t working.”  As Theresa May, the cabinet minister responsible for policing, told Parliament, “Policing by consent is the British way, but the police only retain the confidence of the wider community if they are seen to take clear and robust action in the face of open criminality.” She added later that often “officers are damned if they do and damned if they don’t,” in cracking down on protests and riots.  The broader question, though, is this: How did a national institution once held in esteem, or at least respect, by many Britons — “bobbies on the beat” to an earlier generation — become a force of such contention, even as, in recent years, it has taken credit for shielding the country from an array of terrorist plots?  With their tall, rounded helmets, British police officers have traditionally avoided shows of strength in favor of community policing. Most British officers still do not carry guns.  Yet, in recent years the force, overwhelmingly white, has faced accusations of racism, brutality and incompetence that it has struggled to shake off.  The story of the change in their place is one of incremental missteps as the nature of society itself has changed. Britain has become more diverse and complex in an era when the smash-and-grab crime of earlier decades has been replaced by far more sophisticated law-breaking. And at the same time, the point of contact between the police and the people has been increasingly shaded with questions of race and resentment.  During the latest riots, the most violent in memory, officers were overrun by nimble, well-connected rioters using bicycles, mopeds, smartphones and social networking to organize violence and theft across the capital. At several of the riots, young men could be seen whizzing away from slowly massing riot police officers, only to gather elsewhere.  Across London, as people surveyed the wreckage of their homes and businesses, many complained that the police had done too little. On Wednesday Mr. Cameron announced new plans that would allow the British police to deploy water cannons and plastic bullets against any new violence. The Associated Press reported that facial recognition software would be used to identify looters and rioters captured on CCTV and other cameras. And Mr. Cameron added Thursday that he wanted to discuss going “further in getting to grips with gangs with people like Bill Bratton, former commissioner of police in New York and Los Angeles.”  A former senior riot police officer with knowledge of current operations, who spoke on the condition of anonymity, said that the most recent riots were allowed to rage, in part, because police officers felt constrained. They operated, the former officer said, in the shadow of the case of a newspaper vendor, Ian Tomlinson, who died after being shoved by a riot officer guarding against protesters at a Group of 20 economic conference in 2009. The police officer, Simon Harwood, will go on trial for manslaughter in October.  The Op-Ed columnist Roger Cohen talks about the possible motivations of Britain's disgruntled youth.  Riot police officers in the elite Territorial Support Group, who wear distinctive boiler suits and have been present at most of the unrest, “know they’ve got to be very careful,” the former officer said. “Everyone is filming everyone, and you don’t want to be locked up.” Water cannons and plastic bullets, the former officer said, are “more for P.R. among politicians,” and probably would not be effective against the fast-moving rioters who outran the police. A spokesman for the Metropolitan Police quoted the acting head of the force, Tim Godwin, and said, “We police by consent, that’s the situation that’s always been and it will continue that way.”  Jules Carey, a solicitor for the Ian Tomlinson Family Campaign, which pushed for a prosecution in his death, said by e-mail that the police should restore order “using lawful and proportionate force.”  Others suggested that more aggressive policing to control riots would lead only to further problems. “The ones who are going to get beaten up,” said Rob Berkeley, director of Runnymede Trust, a research organization that focuses on racial equality, “are young people from minority communities.”  Despite some efforts to hire more diverse officers, many people still feel they are “underprotected and oversurveyed,” Mr. Berkeley said, with extensive police stop-and-search powers a particular area of tension. “If you police by consent, trust is crucial,” Mr. Berkeley said. “And these communities don’t trust the police to protect them, or to treat them fairly.”  Metropolitan Police figures from May 2011, the most recent available, show that three times as many black and Asian people were stopped and searched in London as all other ethnicities combined, even though they make up only a quarter of the city’s population. The Metropolitan Police spokesman said that officers operated within national guidelines on stop and search, and that they had to have a suspicion of crime before conducting such searches. He said that 9.6 percent of officers were now black or minority ethnic, and that the police remained “committed to having a diverse work force.”  But at the riots in Tottenham and Hackney in recent days, several rioters cited a hatred of the police, and their perceived racism, as motivating the violence. One young man in Hackney shouted at the officers: “You know you all racist! You know it.”  An earlier version of this article referred incorrectly to the year in which a black teenager was killed. It was 1993, not 1997.  A version of this article appears in print on August 12, 2011, on Page A8 of the New York edition with the headline: British Police Are Assailed by Politicians and Protesters, for Different Reasons. Order Reprints| Today's Paper|Subscribe
The rioting in Britain has turned into a challenge to state authority and, for the police itself, one more entry in a catalog of crisis and condemnation.