Tactic used by police, in which protests are smashed by police coercing protesters into a small space and then keeping them there by surrounding them and attacking anyone who tries to break out or asks to leave. Normally combined with verbal abuse and periodic baton charges against the protesters. It is basically a form of inhumane and extrajudicial imprisonment.
Those held in pigpens often include not only protesters of all descriptions (and not only the ones cops claim to be targeting), and also anyone else who's on the scene, e.g. tourists, journalists, shoppers, local residents.
The tactic has been used fairly regularly by British police against anti-capitalist protesters at events such as Mayday, Disarm DSEi and also at animal rights protests (all of which rely on small numbers of mobile people in small groups to cause disruption).
Referred to as "corralling" in pig-speak; pigpen is the usual term used by protesters. I've also heard it referred to as a kettle, because it's identical to the tactic by this name which was used by the Russian army in Stalingrad.
On Mayday 2001, protesters and assorted others were kept in a pigpen for over 8 hours (with the exception of a large contingent who, led by the White Overall contingent, the Wombles, managed to break out). There were no toilets, no proper medical facilities and it was pouring with rain, and cops made periodic attacks on the crowd, forcing people closer together. There was a real danger of a Hillsborough-style crush, although the real purpose of the attacks seems to be to get "riot" footage to put on the news, so as to make excuses for the cop violence.
Those oppressed by the pigs in this way included also a large number of shoppers on Oxford Street, workers who were returning home, and even two German tourists who cops wouldn't let leave even when they showed their plane tickets - they missed their flight as a result.