Pension reform started the demonstrations in this case, not COVID measures.
"La búsqueda en Google de la frase 'peleas entre manifestantes y policías en Buenos Aires en la Plaza del Congreso” llevó a un vídeo del 18 de diciembre de 2017 que muestra el mismo evento pero desde otros ángulos.'
A search in Google using the phrase "fights between protesters and police in Buenos Aires in the Plaza of Congress" turned up a video from 18 December 2017 that shows the same event but from different angles.
This video, being very violent has made the rounds numerous times, each time promoted as something other than what it was. In fact, it was also ascribed to be from Catalonia - which too is false. I credit WildHighlander with getting me to the right timeframe (past) and used my Spanish to search this out. Translation is mine.