I'll also reccomend being wary of the police...
While hopefully the Canadian police are less inclined to gratuitous violence than, say, Italian or U.S. police, all police forces in the World seem to act irrationally and over-energetically under stress.

Also beware of the "provocateur" type demonstrators (more than likely masked), who may be demonstrators, police (good evidence points to that sometimes being the case during the 2001 Genova G7) or who knows what... They'll probably take great exception to being photographed whatever the case.

Tear gas is just discomfort... Keep your head.