Boca Juniors' team bus came under fire from River Plate fans on its way to El Monumental for the Copa Libertadores final second leg.
The fierce rivals are set to play in one of the most hotly contested matches in their history on Saturday, with the sides level at 2-2 after the first leg.
Boca fans gathered in numbers to wave their players off to the other side of the city, but the reception was nowhere near as friendly on their arrival.
Alarming images showed the bus being hit by objects, while there were suggestions some of the players were affected by the use of pepper spray as police tried to control the situation.
The clash is now set to kick off at 22:15 GMT.