The High Court has ruled that kettling tactics at the G20 protests in 2009 was illegal. But the Police say that kettling "is a necessary tactic to tackle the potential for violence".
Potential is an interesting word. Crowd control is a fine line between structure and chaos.
But that line isn't one created or maintained by the Police. It's one that manifests through trust.
Public demonstration is inherently problematic - if an issue has reached this stage, then there's already a lot of anger around. A lot of people who feel like they haven't been listened to. Of course the potential for violence exists in a demonstration - "successful" politics avoids all physical confrontation, walking or fighting.
A demonstration is a final call to be listened to - but more than that. It's a final show of order and solidarity, a challenge for the ruling parties to trust this potential. If the Police - as the on-the-ground representation of the parties - cannot display this trust, and actively suppress this physical demonstration, then of course anger will turn violent.