For years police have filmed protesters at demonstrations to identify potential troublemakers and collect evidence for prosecutions.
Now, with advances in digital technology and mobile phones with cameras and videos, ordinary members of the public are turning the tables on the authorities.
The issue was brought into focus this week with the suspension of two London police officers after footage emerged of apparent excessive force being used during protests against this month's G20 summit in the British capital.
Video taken by a New York fund manager showed an officer shoving a man to the ground minutes before he died of a suspected heart attack. More film taken the next day captured an officer lashing out at a woman who was remonstrating with him.
In the 21st century, everybody now has...a mobile phone that can record instantly exactly what is happening, Keith Vaz, chairman of the British parliament's Home Affairs Committee, told BBC TV.
Therefore it is not possible for someone just to get away with unacceptable behavior. It is instantly captured on film. You can't in a sense hide anything these days.
The London incidents are the latest examples of how technology is being seized on to bring those in authority to account who might otherwise have escaped justice.
In December, a Greek court convicted eight police officers for beating up a Cypriot student in the northern city of Thessaloniki in 2006 after they were caught on camera by media.
A year later a Greek police officer was suspended after a mobile phone video showing two Albanian detainees being struck with a cane and forced to hit each other appeared on media and video sharing websites such as YouTube.
In the Israeli-occupied West Bank, one human rights group has begun distributing small video cameras to Palestinians to document attacks by Jewish settlers and abuses by Israeli soldiers and policemen.
The scheme has led to the Israeli military charging a unit commander and a soldier with unworthy conduct after a rubber bullet was fired at point-blank range at a bound and blindfolded Palestinian detainee during a protest in the West Bank.
POLICE WILL HAVE TO ADJUST
Professor Peter Waddington, from Britain's Wolverhampton University, said technology had previously driven changes in police tactics, and the proliferation of mobile phones with cameras was the latest innovator.
This has been an incremental process. The reality is we are a surveillance society, but this is true as much for police officers as anyone else, he told Reuters.
We are further up the slope of public surveillance of police actions now thanks to handheld mobile phones. The police will have to adjust to that.
Waddington helped to develop kettling, where police enclose protesters in a confined space, a tactic that replaced the use of horses or crowd charges by lines of baton-wielding officers to disperse demonstrations.
BANANA SKIN PAVING
He said there might now be attempts to prevent police having any close contact with protesters, instead using methods such as banana skin paving, where slippery material is sprayed on the ground to make it impassable.
Whilst the advantages are that police tactics are more controlled, this would not prevent innocent people being caught up in police action -- which they always are in mass disorder.
London police chief Paul Stephenson has already ordered a review of the use of kettling, saying he wanted to be reassured that the use of this tactic remains appropriate and proportionate.
However, David Murakami Wood from the Surveillance Studies Network, a group of academics, said he suspected the focus on police practices might not last, even if it temporarily resulted in greater scrutiny.
Britain already leads the way in the use of surveillance, with a rough estimate suggesting there are at least 4 million closed circuit television (CCTV) cameras on buildings, shops, and stations, and the government is seeking more powers.
It has said it wants to collect more data about the public's phone calls and emails to help fight terrorism.
David Omand, the government's former Security and Intelligence Coordinator, told Reuters last month that law enforcement agencies should have the power to examine the private data of the entire population.
Meanwhile in February, the government brought in measures to restrict the ability of the public to photograph police and security services if the photos were likely to be useful to a person committing or preparing an act of terrorism.
Human rights campaigners say that could give the authorities the right to stop media or the public recording police behavior at protests.
Murakami Wood said photographers already faced increasing harassment, and no other country was trying to control photography and video in the same way Britain was.
It is looking more totalitarian, whether that's what the government wants to call it or not, he said.
But Waddington said the new legislation would not stop people taking pictures at demonstrations, even if police interpreted the law in such a way. Nor would any court prohibit their publication in cases such as the G20 protest incidents.
They can't physically confiscate every mobile phone, he said.
(Additional reporting by Renee Maltezou in Athens and Joseph Nasr in Jerusalem)