Police clashed with Brazilians protesting a visit by President George W Bush and his push for an ethanol energy alliance, while dozens of students in Colombia showed their opposition by lobbing rocks and explosives at authorities.
Violence in Sao Paulo took place several hours before Bush arrived in South America's largest city on the first stop of his five-nation Latin America tour.
More than 6,000 students, environmentalists and left-leaning Brazilians held a largely peaceful march through the financial heart of Brazil before police fired tear gas at protesters and beat them with batons. Hundreds fled and ducked into businesses to avoid the chaos, some of them bloodied.
Authorities did not immediately report any injuries, but Brazilian media said at least six people were hurt and news photographs showed injured people being carried away.
Protesters said scuffles broke out when some radical demonstrators provoked officers and threw sticks at them but said police overreacted. After the clash, the protest continued peacefully but with far fewer people.
The marchers waved communist flags and railed against Mr Bush, the war in Iraq and the ethanol proposal. Almost all had departed by sundown and streets were calm several hours later when Bush arrived in Sao Paulo.
In the southern Brazilian city of Porto Alegre, more than 500 people yelled "Get Out, Imperialist!" as they marched to a Citigroup bank branch and burned an effigy of Mr Bush.
Protesters also targeted the US Consulate in Rio de Janeiro, splattering it with bright red paint meant to signify blood.
In Colombia, about 200 masked students at Bogota's National University clashed with 300 anti-riot police carrying shields and helmets, spray-painting anti-US slogans on walls and shouting "Out Bush!"
Police fired water cannons and tear gas, and the students hurled back rocks, fireworks, a few Molotov cocktails and dozens of "potato bombs" small explosives made of gunpowder wrapped in foil. There were no immediate reports of injuries or arrests.
AP