While this is actually a news story from Great Britain covered by the BBC, I'd suggest it relevant to us here in the U.S. as well.

There are a couple of video clips embedded in the story, so I'm going to post a line or two from the text and ask you to follow the outbound link to read the rest of the story and watch th embedded clips.

Quote:
Police are using hundreds of paid informants inside protest groups to "feed them intelligence", according to evidence handed to the Guardian.

http://news.bbc.co.uk/2/hi/uk_news/8017896.stm