alt Hacker NewsEither. Where there is a need.
The latter is legitimate, isn't it? Do we not anticipate trying to arrest people who might be in the process of committing a terror attack, on the balance of evidence about their conduct? Or are we just going to wait?
Not to say that hasn't gone wrong (infamously and tragically, although that was largely a failure of conventional surveillance) but one key part of mass surveillance policing is arresting people who are clearly about to do a thing.
That might indeed be disruptive protest where it is illegal — for example if you know someone is intending to deface a painting or a monument, or has been planning to attend a protest to disrupt it.
I think people look at face detection policing in this context and imagine thoughtcrime, when the reality is e.g. finding a disturbed person who has been doing a dry run on pushing a stranger off a tube train platform (a real problem that seems to happen every couple of years), catching someone who has been reported sexually assaulting fellow passengers or who has violently assaulted another passenger or someone above ground (a regular problem), trying to disrupt county-lines drug distribution etc. (absolutely constant), or, heaven help us, finding someone who has been spotted at a national railway station with someone on terror watchlists who has then disappeared into the underground.
London Underground isn't a thing that can be effectively managed with ordinary policing. It has 272 stations, over 70 in central London alone. During rush hour there are up to half a million people in that system at any one time. Tube exits are choke points.
I don't know English law, and I'm american so I have a bent towards protecting freedoms. That said, no I don't see it as necessary to arrest someone before they do anything wrong.
There's a gray area for sure, and in the US you can, for example, be charged with conspiracy to commit murder without actually getting to that end goal. I'm always uncomfortable with how a court decides to allow a trial in such a case, though if someone has actively been surveiling their target, collecting materials or weapons, and ideally documenting their intentions somehow then I do understand why most would consider that itself punishable.
If a person without a warrant gets off the tube and walks in a general direction of a protest, I wouldn't see any grounds for arrest. I'm sure there's still gray area there, say they're carrying smoke grenades or flash bangs or something - though in that case I'd assume the potential crime is possessing those items rather than walking in a direction.