Troublemakers marred a gathering in downtown Buenos Aires in support of Argentina's gutsy performance in a 1-0 loss to Germany in the World Cup finals.
Clashes erupted between youths and the police late Sunday near the city's Obelisk monument, hours after what started as a peaceful rally.
Police used tear gas to disperse the rock-throwing crowd.
Officials say at least 15 police officers were injured in the skirmishes.
Police arrested more than 50 people.