Quis custodiet ipsos custodes –who will watch the watchmen? We will, it turns out, with cell phones and digital cameras, with Flickr, YouTube, Indymedia and, on this occasion, some help from the old media in the form of the Guardian newspaper.

The death of newspaper vendor Ian Tomlinson on his way home through this month’s G20 protests in London was dismissed at first by the police as an unfortunate coincidence; a post mortem showed that he had died of a heart attack. Protesters, officers claimed, had pelted them with bottles as they tried to give first aid. Those of us who experienced the police’s tactics on that day–the "kettling" of demonstrators, journalists and passers by, the baton charges in full riot gear against peaceful protesters, the German shepherds straining at the leash–found it hard to credit this quick and anodyne explanation. Three days later the Guardian printed photographs of Tomlinson lying at the feet of riot officers with testimony from named witnesses who had seen him being attacked. The Independent Police Complaints Commission criticised the paper for upsetting Tomlinson’s family.

Then, almost a week after Tomlinson’s death, the Guardian published on its website footage shot by–of all things–a New York fund manager who was there out of curiosity, which clearly shows Tomlinson being pushed roughly to the ground from behind by an officer in riot gear. The video put a girdle round the earth in less than forty minutes. Overnight, the IPCC took control of the investigation. A new post mortem was ordered; the policeman in the video eventually came forward and was–eventually–suspended, though he has not yet been questioned.

Meanwhile, the photos and witness accounts of police violence against protesters and journalists have multiplied. The Guardian has put together a dossier; human rights lawyers are assembling evidence. Senior police officials have been ordered to justify their tactics at the protest. It’s beginning to look as though this story–which some have called Britain’s Rodney King moment–may not end with a cathartic scapegoating of the one unlucky officer whose victim happened to die. Numbed by the memory and threat of terrorism, like the proverbial frog in boiling water, we’ve got too used to living in a surveillance state. In February it became illegal to take pictures of police which might "be useful to a person committing or preparing an act of terrorism"–i.e. pretty much any pictures of the police. The aftermath of Ian Tomlinson’s death has proved that law unenforceable, and showed what can be done if enough citizens turn their cameras on the watchmen.