The attack on Ian Tomlinson was the Metropolitan Police's "Rodney King moment". King, you may recall, is a black American who, in March 1991, was savagely beaten by Los Angeles Police Department officers after being stopped for a speeding offence. A resident videotaped the proceedings from his apartment. The Los Angeles District Attorney charged four officers with use of excessive force. A jury acquitted three of them and failed to agree about a verdict on the fourth. Six days of rioting followed, in which more than 50 people died and $1bn of property was destroyed.

The assault on Tomlinson will not spark off a riot, but nobody should underestimate the outrage it has generated. And from the instant the video footage - shot by an American bystander using his digital still camera - appeared on the Guardian website, it was clear that we had reached a pivotal moment. Consumer technology had given citizens a serious tool for recording how policemen behave.

It also brought to mind the case of Blair Peach, the young New Zealand teacher who, on a demonstration 30 years ago, was clubbed by a police officer and died the day after of his injuries. Nobody was ever tried for the assault and the coroner recorded a verdict of "death by misadventure".

There was no "citizen journalism" at the time of the Peach case. Nobody had a cameraphone or a digital camcorder, because they hadn't been invented. And the incident wasn't recorded by any press photographer or film crew. So the cop who attacked the young teacher escaped scot-free.

In a normal democracy we would expect that the technology which revealed what really happened to Tomlinson would stimulate a reassessment by the police about how they conduct themselves. Accidents will happen, terrible things are sometimes done in the heat of the moment, and political demonstrations attract their share of violent and disturbed people, but from now on the police will have to reckon with the possibility that anything they do will be recorded and globally published. At one time, they - and the authorities they serve - were the only ones with CCTV and face-recognition technology, the ones with the sole prerogative to videotape and photograph demonstrators. Now this technology is in the hands of consumers.

The police have two choices. Accept that digital technology will make them accountable for their actions or try to control the technology. In any normal society there would be no decision to be made. But since 9/11 the threat of global terrorism has given the state - and its security apparatus - carte blanche to take whatever measures it deems necessary. And it has imbued in every uniformed operative, from "Community Support" officers and the bobby on the beat to the bored guy in the airport checking your toothpaste, the kind of arrogance we once associated only with authoritarian regimes.

You think I jest? Talk to any keen amateur photographer. As a group, photographers have been subjected to increasingly outrageous harassment by police and security operatives. (For a partial list of incidents see bit.ly/22VFRX). Try photographing a bridge, public building or a police car parked on a double-yellow line and you will have a goon demanding your camera, image card or film.

Better still, ask John Randall, a Tory MP who recently told the Commons how one of his Uxbridge constituents, a Mr Wusche, photographed properties he thought were in bad repair to pass on to the council. In front of one building was a police car containing police community support officers who had parked on a double yellow line as they popped into a sandwich bar.

Randall told MPs that "one of the PCSOs went over to Mr Wusche" - who fled fascist Italy in his youth - "and told him that he must immediately delete the photographs. When Mr Wusche asked why, he was handed a notice and pretty much cautioned. That upset him a great deal".

It upsets me too. And I expect that when the fuss over Ian Tomlinson's tragic death has died down, we will find that the Nokia N82 and the Canon Digital Ixus have joined flick-knives, knuckledusters and coshes on the list of "offensive weapons". Welcome to New Labour's National Surveillance State.