Four River Plate players have been taken to hospital after being sprayed with an irritant by Boca Juniors fans in an abandoned Copa Libertadores tie.
The last-16 second leg in Buenos Aires, Argentina, was called off at 0-0 after the players were attacked as they came out of the tunnel after half-time.
River manager Marcelo Gallardo described the scenes at Boca's La Bombonera stadium as "deplorable".
Boca president Daniel Angelici said: "We will do all we can to help police."
Fans managed to get through high wire fencing and on to the pitch.
Television pictures showed River players Leonardo Ponzio and Leonel Vangioni rubbing their eyes, with other players pouring water over their faces after apparently being sprayed with a pepper spray-like substance.
"I can't see, I can't see. I am burning. This is not a war," said River Plate defender Ramiro Funes Mori during chaotic scenes as the match was halted.
All three players, along with team-mate Matias Kranevitter, were taken to hospital for treatment for what Argentine radio described as burns.
Angelici said his club would "accept responsibility" for the incident but added Boca's players should not be punished for what he said was "society's problem, not football's problem".
"These people are sick," he added. "An aerosol is small, it is very powerful. It can fit in your pocket and when there are almost 50,000 people it can get by security."
South American football's governing body Conmebol said its disciplinary committee has started disciplinary proceedings against Boca and given the club until Saturday to respond.
The association still has to decide whether to replay the match, complete it at a neutral venue, or award the tie to River, who were 1-0 up from the first leg on 7 May.