Critics of the spread of closed-circuit television, government databases and intercept techniques have long been complaining about the surveillance society. The Big Brother state has, we have been told, encroached on every aspect of our private lives — criminalising us for putting the wrong rubbish in the wrong bin, spying on what school we send our children to and collating details of our e-mails.

We are becoming a “police state”, or so the people protesting during the G20 summit would have us believe. But those protesters do not appear to have realised that they have turned the tables on the prying state.

Violent demonstrators were caught on camera, but the images of criminality allegedly perpetrated by police officers are causing more of a stir. Journalists, protesters and passers-by photographed and filmed what appear to be acts of police violence against people who posed little threat. Police officers have been suspended and face possible criminal charges.

Around the City of London, investigators are harvesting CCTV from street and shop cameras to trace Ian Tomlinson’s contact with police before he died.

The Met has ordered a minute-by-minute trawl through its own footage and promised to act if it discovers misconduct by its officers.

The Independent Police Complaints Commission has a unit investigating the alleged assault on Nicky Thomson, which goes under the name “G20 YouTube investigation team”.

At any demonstration these days you will find members of FITwatch — activists dedicated to filming the police’s own camera units, or Forward Intelligence Teams.

Footage from demonstrators has been appearing increasingly on the websites of activists and social networking sites. But apart from those directly involved in the campaigns, no one paid much attention: G20 seems to have changed that.

Films made on mobile phones and posted on YouTube have gone into the mainstream media and become powerful tools for holding police to account. The most senior policeman in the country was forced to respond yesterday and announce a significant review of police tactics and behaviour.

If, in future, police use the controversial section 76 of the new Counter Terrorism Act to stop people taking pictures of officers during demonstrations, that would be a dangerous abuse of power.

Times Recommends

• Foxy Knoxy snared by her lust and her lies
• Britain is the land of the shrinking family
• Zac Goldsmith: Not such a green god