I just now saw this.  I was thinking a march or something and the thought of things descending into actual riots and brutality hadn't occurred to me until I read Metatone's great advice.

With that in mind, this old, scarred veteran of more than a few riots in L.A. only has this to add -- don't ignore the cloth advice!  Listen to Metatone!  Wait, that's not adding anything, it's nagging... sorry.  Okay, take a bottle of water if you can, both to flush your eyes (contacts can only do so much) or to wet the cloth with and breathe through.  

Also, if there's a chance you might end up in jail, write an emergency phone number on your forearm in ink.  It's difficult to remember numbers in these situations, even if you know them well.  One last thing -- BE CAREFUL!  I'll look forward to seeing your photos.  Let us know when you're back safe!

Maybe we can eventually make language a complete impediment to understanding. -Hobbes