"One day in June, 1973, three months before the bombing by Pinochet's military coup, I was walking through the plaza in front of the Palace of Finance in Santiago, Chile, and saw a street singer shouting, "The people united will never be defeated" - a well known Chilean chant for social change. I couldn't stop, and continued across the square, but his incessant chanting followed me and stuck in my mind.

On the following Sunday, after the broadcast of the show "Chile Says No to Civil War", which I directed for Channel 9, we went with a few artists to eat at my house outside Santiago. Upon arrival I sat down at my piano and thought about the experience in the plaza and the events at large. When I reproduced the chant of the people in my head, the chant that could not be restrained, the entire melody exploded from me: I saw it complete and played it in its entirety at once. The text unfurled itself quickly and fell, like falling rocks, upon the melody. In their enthusiasm some of my guests made suggestions that were too rational for the situation I was composing in. Out of courtesy I pretended to accept, but arranged myself to leave the text in its symptomatic landscape.

The song was performed in public two days later by the group Quilapayun in a heavily attended concert in the Alameda.
--Sergio Ortega, on El Pueble Unido"
http://unionsong.com/u443.html 

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I don't know which version it is but I like this on YT. Lots of participation from the audience.
https://youtu.be/Hhhk4oj6nWk 


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The Rzewski is a great listen, though I can't take the whole album at one sitting.