Not good

This incident has been doing the rounds on the blogosphere. And now, as is usual these days, some video footage has come to light.

To me, the issue is actually less the use of CS gas - although this certainly seems to have been disproportionate - but rather the initial arrest. Arresting someone for criminal damage for pushing leaflets through a gap in a door (which is allegedly what transpired) is plainly ridiculous and was bound to increase tension. Events then took their course and, to give him the benefit of the doubt, an officer felt threatened enough to use the spray (less generous assessments certainly suggest themselves, but it's hard to see from the footage).

Whichever way you look at it, this seems to have been a situation created by the police. The range of powers available to the police at demos is very wide, and they don't need to use arrests like this to manage situations, which is what I suspect they thought they were doing (presumably to protect property).

Police have a duty to maintain order that is at least as powerful as their duty to protect property - and it certainly seems to be the case that here they've taken one course that directly affected their ability to maintain order at what seems to have been, up to that point, an entirely peaceful demo.