This video shows 3 "protesters" at a demonstration suspected of being police agents trying to incite violence. This article from The Star (Toronto) gives some background and follow-up.
In the video, I noticed a police officer with a video camera, although he put it out of sight when he noticed that he was himself on camera.
Also, this blog post on rustyidols points out that the "protesters" were still masked when they were led away by police, as confirmed by photos and the video (around minute 3:30 to 4:00). Furthermore, the police do not admit to having arrested the three men, even though the four other arrests made were acknowledged and the arrestees are accounted for (three released, one still in custody at last report).
The article also states that the uniformed police officers' boots and those worn by the "protesters" had identical yellow triangles on the soles; low-resolution photos exist here, but I don't think of this as significant evidence, as these triangles are standard safety sole markings.
This is, to say the least, disturbing. The police seem to be increasingly defending the status quo, seeing anyone who criticizes it as a threat. In order to nullify that perceived threat, they appear to be willing to undermine the vital political discourse of protest and demonstration and deceive citizens in order to build a case for putting people away.
It wouldn't be the first time the police profile protesters as likely offenders and attempt preemptive measures designed to trigger illegal activity in a "controlled" situation. Because they are an arm of the government, however, they are structurally unable to do this in a politically neutral way. As a result, movements for political change are unjustly suppressed and stifled, and the status quo is preserved beyond its viability. Because real public opinion remains invisible, changes are delayed until they are liable to be explosive, with the attendant disruption and possible violence.
If police currently use undercover provocateurs at political demonstrations and protests, they must stop doing so. The short-term gains they might achieve in maintaining "law and order" are too likely to be greatly outweighed by the resulting long-term loss of respect for the law and the police, and by the violent eruption of voices too long silenced.