These new age brown shirts aren't confined to this event. They (ne'er-do-wells dressed in black intent on rioting and inflicting property damage in the name of the cause-du-jure) make appearances at other evens such as meetings of the WTO and G8 in the US and Europe. They have also announced plans on the internet to disrupt the Republican National Convention, as well. They are fuzzy on their ideology; these gangs are usually made up of an odd alliance of communists and anarchists. Their overarching grievance is usually euphemistically phrased as "anti-imperialism" or "anti-capitalism" of course we see in this case, the real result of their actions is to interefere with the legitimate democratic process.
Blogger and citizen journalist, Zombie, was in Denver and has reports posted on littlegreenfootballs. Follow the links for pictures and excellent commentary:
Zombie: The Denver Games - Opening Ceremony
Zombie: Recreate 68 Finally Lives Up to Its Name: Riot in Denver
An unfortunate side effect of announcing your riot on the Internet is that the police can see the announcement as well. As a result, squads of cops milled through the crowd, looking for troublemakers.