It is pretty well known in advance where these "rallies" will be taking place.  Activate the Guard forces 24 hours prior and deploy them to the scene.  If or/when trouble begins they should wade in and start breaking heads.  Police forces should be over aggressive and arrest any and all for even minor infractions and those arrested especially for a felony be tried and if convicted served the maximum time allowable for the offense and be required to make restitution for any and all damages they and their group inflicted.