So successful was the kettling tactic it was implemented at all future potentially inflammatory ‘mass demonstrations’ as a method of controlling, subduing and ultimately criminalising protestors. (Early kettling procedure went hand in hand with the misuse of ’section 60′ order whereby police took the personal details and pictures of all those in the kettle for their database).

Although not defined in law (it still is simply a police tactic) it was given the green light by the High Court after some protestors questioned the legality of their seven hour kettling in Oxford Circus on Mayday 2001. The courts ruled that the police could under certain circumstances detain people against their will for long periods of time to prevent outbreaks of violence and criminal acts – and typically a breach of the peace. The example they gave was the detaining of football supporters in the ground while opposing fans left the area. The question of the fact most of the 2,000 people detained in Oxford Circus were in fact law abiding was described by the judge as “unfortunate”.

In terms of the practicalities of the law it’s an absolute sham and the judges who made the ruling know they got away with a large dose of bullshit. But it also meant the police could continue the tactic along with all the violence and thuggery that goes with it under the protection of the law.

The anarchist group the Wombles developed a strategy (borrowed heavily from the Italian radical left movement Tute Bianche) of wearing padding and protective headgear and using re-enforced banners to break through police lines on demonstrations.

While this proved very effective in the short term – the police had no idea how to handle such a disregard from their authority, it had to rely on everyone else adopting the same strategy of forcefully breaking out of the kettle to be truly successful. In the end it was seen as vanguardist (by the left, without a hint of irony) or too specialised to be universally adopted by everyone.
 http://www.wombles.org.uk/article20060318.php

The real purpose of the kettle is to ensure people are dissuaded enough not to attend future protests – the law of diminishing returns – which actually succeeded throughout much of the 2000s. This generation of protestors from what we’ve seen, aren’t going to be that easily put off.

Implications of “kettling” and the contradiction of European human rights legislation:
 http://www.criminallawandjustice.co.uk/index.php?/Analysis/public-order-policing-and-the-echr.html