During their match at the Estadio Nacional Julio Martinez Prádanos, fan groups disrupted the game. Supporters from the 'Garra Blanca', Colo Colo's ultras group, had been disruptive before kickoff but caused pandemonium when they attempted to enter the pitch during the second half.
According to Chilean news site La Tercera [via The Daily Mail], the Garra Blanca 'invaded the athletics track' on the pitch's outer edge, intending to enter the field. As police wrestled with supporters to keep them off the ground, things at the stadium swiftly became ugly, with flares and pyrotechnics let off in the stands.