At the park across from the Ministerio de Educacion (b/n Callao y Rodriguez Pena & Paraguay y Marcelo T. Alvear) there is a group that meets for Tai Chi in the mornings-ish. ~10:00am.
The first time I saw it was something I'll never remember.
There is a dog park thing (fenced in area for dogs) in that park. There were about 30 dogs barking like crazy with one or two spectacularly insane dogs rolling around in a mud puddle like they were piglets on crack... and if you turned your head about 30 degree to the south, there were about 20 blue hairs going through the motions completely oblivious to the Call of the Wild just about 15 to 20 meters away.
I think that I could even hear the soft notes of Asian (Chinese?) music occasionally detectable over the barking of the dogs. My guess is that most of the participants had their hearing aids turned down to the "Serenity Now" setting.
There is a common misconception that Tai Chi is only for "blue hairs" as I have been taking Tai Chi on and off for over 10 years, I am 45 and 80% of the classes are filled with teens and twenty-somethings, mostly female.
That being said I go to the "Centro San Ti" School that is at Estado de Israel 4116 and pay 70 pesos to take 2 classes a week for a month. They teach Tai Chi Chuan, Kung Fu and Aikido. A number of the students and the Aikido professor speak perfect english.