River Plate fans celebrated after their Copa Libertadores victory over Boca Juniors.
The fierce rivals finally played the second leg of their final in Madrid on Sunday night as River emerged 3-1 winners after added time.
The match had to be moved due to violence in Buenos Aires but passed with no trouble.
But there were ugly scenes back in the Argentine capital as fans clashed with riot police in the early hours.
Police were forced to disperse large crowds and were seen detaining some supporters after bottles and other objects were thrown.