By sheer coincidence, I bought Marc-Andre Hamelin's recording of Frederic Rzewski's piano variations on this song yesterday - "The People United Will Never Be Defeated" (Hyperion CDA67077, 1999) from the Christian Aid book sale in Edinburgh. It's probably Rzewski's best known piece. I heard him playing here a few years ago; phemomenal performer and I think he's still at it. (The sleeve notes don't add much to what Joe said; apparently Ortega built his song around a street chant he heard in early 1973).