Did they do this by beating down doors? No, they sent a letter to those with form, those previously arrested on public disorder grounds in similar circumstance to those of today's impending protest.
The letter says: "It is in the public and your own interest that you do not involve yourself in any type of criminal or antisocial behaviour. We have a responsibility to deliver a safe protest which protects residents, tourists, commuters, protesters and the wider community. Should you do so we will at the earliest opportunity arrest and place you before the court." Sounds pretty fair warning to me. Start lobbing missiles, intimidating others and inhibiting the normal functioning of the city and we'll have you.
Surely this is a pretty reasonable bit of proactive policing? Not according to the
Aw, diddums. Well, Michael, old chum, that isn't actually their job. Their job is protect law-abiding citizens from the pseudo-anarchic activities of some of the idiots who will hijack your lost cause for their own mayhem-making merriment. I'd love to hear your opinion on all of this once you have paid taxes for a few years, but right now your right to protest is an indulgence that many countries would not afford you. Why not belt up, just get on with it and try not to set anything alight today, okay?