River Plate fans appeared to be responsible for an ambush on Boca's team bus, which left one player injured and unable to play.
Met by a barrage of River Plate fans, the Boca Juniors bus was pelted with missiles and, crucially, tear gas.
After Conmebol's original decision just to delay the match, Boca's former Manchester City forward Carlos Tevez told Fox television: "We're not in a condition to play".
Videos emerging in the immediate aftermath outlined the destruction that had been brought upon the Boca bus, and the vulnerability of the players on-board as a result.
The game was set to be contested River Plate's Estadio Monumental Antonio Vespucio Liberti (El Monumental) stadium with the two sides level at 2-2 after the first leg.
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After several postponements saw kick off time pushed back further and further, South American football federation CONMEBOL confirmed the match would be postponed until Sunday.
According to local media reports, tear gas made its way into the Boca Junior bus through the broken windows, leading to the players having trouble breathing.
There were unconfirmed reports of six Boca players vomiting due to pepper spray inhalation - with claims it was River fans, not the police, who used the gas.
Despite the damage done to the bus, and the injuries suffered, Conmebol was reported to be forcing the game to go ahead.
He had also promised that the move to a one-off final from next year would be safe, but after this chaos, the thought of what might unfold were tens of thousands of Argentinians supporting rival clubs to make the journey west to Santiago, where next year's showpiece will be held, is deeply disturbing.
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The full document read: "I'm writing to you to inform about today's incidents in which Boca players suffered superficial skin injuries to their arms, legs, face and torso, with two players [those sent to hospital] reportedly suffering eye injuries which our medical team has not been able to confirm".
There will be no Boca fans at the second leg as travelling support has been banned at all Argentine football matches since 2013 due to hooliganism.
The two teams agreed to play Sunday evening instead of delaying the match start a few hours as originally planned, the president of the South American Football Confederation told reporters.
This local encounter is widely viewed as the fiercest rivalry in world football. River Plate won the title in 1986, 1996 and 2015.
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