Word Spy reports on the use of the word kettle to describe police surrounding demonstrators:
v. To maneuver protesters into a small area using a cordon of police personnel and vehicles. —n.
Here’s an example from the Guardian on March 30:
But it wasn’t really their natty dress-sense or their flags that made them stand out so distinctively from the rest of the march. It was the very special policing tactics that were focused on them: the anarchists, the police seemed to feel, were such an imminent danger to society that they needed to be ‘kettled’ — in other words, to have three police vans crawling along blocking their left-hand side, and a tight line of police one behind another on their right-hand-side, to make sure there was no possibility of break-out.
The word einkesseln is more familiar in German. Spiegel Online, under a heading Einkesselung bei Anti-Rechts-Demo:
Am frühen Nachmittag beginnen die Einsatzkräfte, “Schließungen” durchzuführen – also große Gruppen zu umzingeln. Gleich drei solcher Kessel richten die Einsatzkräfte ein. Damit habe man circa 500 gewalttätige Störer wegen Landfriedensbruch festsetzen wollen, so die Kölner Einsatzleitung. Unter den Eingeschlossenen befinden sich neben zahlreichen Linksautonomen auch drei Kinder und 72 Jugendliche.
(These demonstrators were later taken to Brühl and put into open cages, but that’s a different procedure).
Other words mentioned by Word Spy are globophobe, pickade and summit-hop.