In response to this thread, where the Quebec cops planted rock-toting undercover cops at a protest:
http://www.democraticunderground.com/discuss/duboard.php?az=view_all&address=389x1671292I'd say it's perfectly acceptable to knock the bastards down (or out) and hogtie them, then hand them over to their fellow cops. These guys are going to be the excuse for the worst elements of the police force (since riot cops seem to be the ones most likely to be corrupt/in it for the violence) to abuse their power and authority, and quite possibly get people severely injured or killed in the resulting swarm of baton-wielding cops. Additionally, they're violating your right to free speech/assembly. It's hardly "free" if the authorities have someone there to give them an excuse to crack down on you. As far as I'm concerned, that means you have carte blanche to knock their corrupt, un-American asses out.
The police shouldn't be able to legally press charges against you or rush the crowd (although the legal aspects may not stop them), because you can say something to the effect of: "I was just making a citizen's arrest and saving you the trouble, officers. How could
I know he was an undercover cop?" Just yell "Citizen's Arrest!" as you drop the guy, and you ought to be fine.
And in the (unlikely) event of subduing a legitimate (if imbecilic) protester -- well, things haven't quite gotten bad enough that everyone should be taking up whatever weapons they can get hold of. Peaceful protests still have a chance to make an impact, so anyone stupid enough to try violence at this point in time needs to learn a lesson.