Reports said Boca's Pablo Perez and Gonzalo Lamardo had been taken by ambulance to hospital, the former with slivers of broken glass in his eyes and the latter a cut on his head.
The rivalry is one of the biggest in football - with both sides originating from the La Boca neighbourhood before River relocated to a different district of Buenos Aires. The bus was then subjected to a hail of objects, which succeeded in breaking at least three windows.
Independent reporter Miguel Delaney tweeted: "Pepper spray got onto Boca bus after River fans attacked it, and police tried to disperse, leaving players wretching".
"They were throwing pepper gas, stones, everything", the Clarin website quoted Juan Carlos Crespi, a member of the Boca delegation, as saying.
Further reports suggest that the driver of the Boca Juniors bus almost collapsed upon arriving at the Monumental due to the effect of the gases, while some of the players were said to be vomiting, with Tévez, Wanchope y Espìnoza among the worst affected.
A riot policeman fires his gun after the match was postponed
"Boca players apparently want to play, though some visibly wretching coming in". As the driver added: "For me it was a free-fire zone, we were coming to a football match, not to war".
Television pictures showed Boca players emerge from the bus clearing their throats and with eyes streaming from the effect of the gas.
But a statement released by CONMEBOL this evening says the clash has been postponed and will be played at 7.30pm local time, rather than suspended until further notice. On that occasion, River were awarded the game.
All the while thousands of River supporters sat in the stands in nervous ignorance at what was going on in the bowels of the Monumental.
Meanwhile, 66,000 River Plate fans were in the stadium waiting for a match that has been one of the most anticipated in Argentine football history.
"The team which loses will carry a cross for the rest of their life", was how Marcelo Matador Salas, the Chilean striker who used to play for River and Lazio, described it. The damage, though, to the spectacle and to the reputation of Argentinian football had already been done.
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