Os Racionais is a popular Hip-Hop group in Brazil. You can check out some of their music on their mysp*ce page. On May 5th they held a concert that was either broken up during or before their performance. Couldn’t find any English articles. You can hear the tear gas guns in the video clip. Here is a google translated account from a Brazilian blogger. The controversy seems to be over whether the break up was premeditated or not. Link (via)