First, go watch some of the videos from Iran from the past week.
Welcome back. Now watch some of these videos showing what the police forces in our own country do when Americans take to the streets.
Seattle - WTO Protest, 1999
http://www.youtube.com/...
http://www.youtube.com/...
Protest of the Free Trade Area of the Americas, Miami, November 2003
http://video.google.com/...
http://www.youtube.com/...
Republican Convention 2008
Republican convention: Police attack peaceful crowd
http://www.youtube.com/...
Amy Goodman Arrested at RNC
http://www.youtube.com/...
Democratic Convention 2008
http://www.youtube.com/...
DNC Drama: ABC Reporter Arrested In Denver!
http://www.youtube.com/...
These are just a few of the first things that came up on Google searches. There's lots more.
Then there are the conventions of 2004, and 2000, and... well, you get the idea.
The lesson: the Establishment in America doesn't like street protests any more than the Establishment in Iran does.