The meeting between Universidad de Chile and Independiente in Buenos Aires was interrupted early in the second half after spectator trouble.
Clashes between the two teams' fans resulted in at least 19 injured, three of them seriously, and over 100 arrested.
Stones, chairs and bottles flew between the supporters in the brawl. There are also reports of people being stabbed.
Chile's president Gabriel Boric is now demanding justice for his compatriots and calls what happened a "unacceptable lynching". Boric is sending the country's interior minister to Argentina to follow the situation on the spot.
Even Fifa's chairman Gianni Infantino has spoken out after the chaos and describes the violence as "barbaric" and calls for "exemplary sanctions".
The clubs involved risk, according to reports, at least fines, but also exclusion from the South American cup is a possibility.
Spectator trouble that escalates to violence with fatal outcome has occurred several times in the past decades in South American football.
As recently as in April, two supporters died after clashes with police in connection with the meeting between Chilean Colo Colo and Brazilian Fortaleza.
