Hundreds of demonstrators clashed with police at the opening day of the Summit of the Americas in Mar del Plata, Argentina.
They fired Molotov cocktails, threw stones and set up burning barricades, protesting against "imperialistic" policies of US President George W Bush, who is attending the summit.
Rioters smashed offices of dozens of businesses, seen as symbols of capitalism.
At least one branch of a bank in the seaside resort was gutted by fire.
In response, riot police fired tear gas to disperse the protesters.
More than 60 people were arrested, and police later said the situation was "under control".
A much bigger anti-US demonstration - attended by thousands of people - passed off peacefully in the town.
Addressing the rally, Venezuelan President Hugo Chavez denounced a free trade agenda championed by Mr Bush and urged the participants to "defeat imperialism".