The Police Are Following You - They Can Digitally Stop and Search You

Privacy International (July 13, 2018) wrote "How would you feel if you were fingerprinted by the police before you were allowed to take part in a peaceful public demonstration?

As tens of thousands of people attend massive public demonstrations across the UK today against US President Donald Trump in a ‘Carnival of Resistance’, it’s a question worth asking. Why? Because the police now deploy a range of highly sophisticated surveillance tools at public events which are just as if not more intrusive. And these technologies should be even more alarming than being fingerprinted, because at least you would know if you were being fingerprinted - the surveillance technologies the police now use are largely invisible and imperceptible.

We want the police’s use of these technologies to be properly regulated so that people can peacefully demonstrate without being inappropriately spied on."

The Electronic Frontier Foundation (EFF) published as part of their Surveillance Self-Defense Guide, tips for activists and protesters to protect their communications.
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I strongly support local law enforcement, and I believe that in almost all cases the cops are the good guys. Police officers in your town, the Sheriff's deputies in your county, have a difficult job safeguarding and protecting our communities against burglars, muggers, thieves, and the like. We ask the police to protect our communities and they need to have the tools needed to do that job.

The problem we face is that police use mass surveillance, data collection, and retention of data on large groups of people (such as political activists) in the hope of finding a bad guy, instead of investigating specific individuals suspected of a crime, or in too many cases in the hope of disrupting political protest and lawful dissent.