Activists are set to rally Tuesday in several US cities demanding justice for slain black teenager Trayvon Martin on the first anniversary of his death. Martin, 17, was shot dead on a rainy night one year ago by George Zimmerman, 29, a volunteer night watchman. Zimmerman had followed Martin into a gated community in Sanford, in central Florida, and said he shot the unarmed teen in self-defense following an altercation.