To maneuver protesters into a small area using a cordon of police personnel and vehicles. —n.
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.
I've seen 'kettling' done even more intensively than this, at a recent Smash EDO protest in Brighton, for example, where the police were lined along the front and back of the group as well as down the sides, and I (walking my kid home from school) was warned to get out of the way as if it was an advancing army, rather than just 50-60 protesters. This small group in the 'kettle' was as resentful as you would expect.
—Bibi van der Zee, "'Kettled' anarchists increase worry for G20 demonstrators," The Guardian, March 30, 2009
Jokes being exchanged with the wall of police in front of the Bank of England seem to have turned sour, with protesters now stealing their hats and passing them across the crowds. "They're kettling us,' a woman shouts, a protester's expression I know now to describe maneuvering crowds into one place.
—Celia Walden, "G20 summit: The day I joined the protesters," The Daily Telegraph, April 2, 2009
We knew there'd be lots of cops in our way. As well as the publicised meeting point, we had other informal meeting places so that people could converge and break through any cordons the police formed. Unfortunately, many people did get 'kettled-in' despite the plan.
—Lydia Molyneaux, "The carnival continues...," from the collection Shut Them Down! (Chapter 9 [PDF]), published by Autonomedia, January 10, 2006
Why kettle? Reader Bernie Williams suggests it comes from the German word kessel, "kettle," and in particular the variant einkesseln, a military term which means "to surround."
Here's a citation for the noun sense of the term:
Well, all four marches made it to the Bank of England - only to be blocked in by police in what's apparently called a 'kettle'. Nobody's being allowed out of the area in scenes reminiscent of Oxford Circus in 2001.
—"G20 Protests: 'Kettle' At Bank, Climate Camp Pitched," Londonist, April 1, 2009