Well, that really sucks! Sooner or later in any major city you will run into things like this. The best things to do is just consider it a lesson learned. While your intentions were good the environment for your intentions was not suitable. Hard to believe and yes, that sucks too... but it's true.
All major cities have rules of engagement. So unless you are armed and trained sufficiently to intervene it is simply not worth it, and even then, you need to be prepared for the out come. Which in Argentina, from what I hear, whacking thugs makes you the criminal.
It will take you a few days to recover from the fear factor and shock of what you experienced. You will most likely find yourself looking over your shoulder a bit more. But life will go on and your senses should be a little keener and you will be a little wiser. I would chalk it up to one of those really crappy life in the big city educational experiences. After all you have a life and the critters you encountered do not and most likely never will.
Also I do not think they will be watching for you or even paying attention to whether you are around or not. Like Lee said.. Just a random encounter.