Summits "attract a group of people who are here for these types of altercations," said John Thompson, a security expert with the nonprofit Mackenzie Institute in Toronto, which studies political instability and organized violence. "This is going to happen no matter what [police] do."
Such groups, which police have taken to calling "black bloc," for the black clothing members often wear, study police tactics to determine how to inflict the most damage without getting caught, Mr. Thompson said. Members tend to have a hodgepodge of political views and attend protests more for "a rush, a thrill" of violence, he said.
"The black bloc is a TACTIC, not a group or organization. Just like there cannot [be] the "Civil Disobedience Group," neither can the black bloc be an organization. Some people are under the mistaken impression that one can join the 'black bloc organization.' There is no standing black bloc organization between protests. There is the anarchist movement which always exists (and has been around for over a century). You can think of the black bloc as just a temporary collection of anarchists that represent a contingent in a protest march. The black bloc is a tactic, similar to civil disobedience."
Black Blocs for Dummies