Boca Juniors fans fired pepper spray at rival River Plate players forcing authorities to halt a Copa Libertadores game in the latest violence to hit world football.
Four River Plate players suffered burns, according to media reports.
Players were returning for the second half of the game, deadlocked at 0-0, when the shocking attack was staged by fans among the 60,000 capacity crowd in Boca's Bombonera stadium.
The South American Football Confederation (CONMEBOL) said it would meet Friday to decide whether to try to finish the game or just kick out Boca Juniors for the conduct of their fans.
River Plate won the first leg of the round of 16 game 1-0.
River Plate players were coming out of the tunnel from the dressing room for the second half when the irritant was sprayed at them.
Team doctor Pedro Hansing told Argentine media that four players were treated for pepper spray burns. Leonardo Ponzio, Leonel Vangioni, Ramiro Funes Mori and Matias Kranevitter were all seen suffering.
"I can't see, I can't see. I am burning. This is not a war!" shouted River Plate defender Mori.
Ponzio's face swelled up and Mori said he had burns on his body and in his eyes. Teammates Tomas Martinez and Vangioni were seen desperately throwing water in his face to try to relieve the pain.
Amid wild scenes with fans hurling rocks, bottles and other missiles, authorities waited for an hour before calling off the match. Fans climbed over a six metre (20 feet) high wire fence to get onto the pitch.
After referee Dario Herrera called off the game, some players ran off with their heads covered in wet towels in case of new attacks. Police guarded others with a tunnel of shields to stop them being hit by projectiles.
Boca Juniors condemned the violence. Boca player Marco Torsiglieri said "what happened today was embarrassing, sad" in an apology to the River Plate players on Twitter.