"Assembled in a crowd, people lose their powers of reasoning and their capacity for moral choice. Their suggestibility is increased to the point where they cease to have any judgment or will of their own. They become very excitable, they lose all sense of individual or collective responsibility, they are subject to sudden accesses of rage, enthusiasm and panic. In a word, a man in a crowd behaves as though he had swallowed a large dose of some powerful intoxicant. He is a victim of what I have called 'herd-poisoning.'"[1]
[1] http://reasontostand.org/archives/2011/01/19/aldous-huxley-o...
Also, for historical interest the blue-green riots in Rome: http://en.wikipedia.org/wiki/Nika_riots