(Audio-only) Riots occurred over multiple days following Dr. King's assassination, in 60 U.S. cities, but not in Indianapolis.
"Spurning his advisors' recommendation to forego the speech, Robert Kennedy informed a shocked crowd about the killing, then spoke extemporaneously about King, and appealed for hope and faith, concluding his speech with the words 'Let us dedicate to ourselves to what the Greeks wrote so many years ago: to tame the savageness of man and make gentle the life of this world. Let us dedicate ourselves to that, and say a prayer for our country and for our people. Thank you very much.' The audience, and the city itself, heeded Kennedy's wish."http://www.funtrivia.com/en/subtopics/The-Assassination-of-Martin-Luther-King-201065.html
I once saw a video of this speech, where the camera man turns the camera but not the audio off midway through out of respect for Dr. King and the crowd around him, who were losing it.