Pigpen refers to the infamous peanuts character who can bearely be seen through the cloud of dust and dirt that follows him. He is the epitome of disgusting.
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.
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.