Chicago police getting ready for rally

I was walking around today and noticed lots and lots of police on every street corner. I thought it might be some terrorist training, but it looks like they're getting ready for some sort of rally in front of the post office. The area around the office is fenced off and there is a stage with sound equipment. I'm guessing they're commemorating the 2nd anniversary of the start of the Iraq war (March 20, 2003). I'll get more info when I go out for lunch. For now, here's a pic of the area: