Bush arrived in South America's largest city in the evening on a mission intended to promote democracy, increased trade and cooperation on alternative fuels. The president and his advisers also hoped his visit would offset the growing influence of leftist leaders, such as Venezuela's Hugo Chavez.
As he flew here on Air Force One, Bush's national security adviser brushed aside Chavez's provocations. "The president is going to do what he's been doing for a long time: talk about a positive agenda," said Stephen Hadley.
Police battled students, environmentalists and left-leaning Brazilians, some waving communist flags, ahead of Bush's visit. Riot police fired tear gas and beat some protesters with batons after more than 6,000 people held a largely peaceful march through the financial district. And in the southern city of Porto Alegre, more than 500 people yelled, "Get out, imperialist!" as they burned an effigy of Bush outside a Citigroup Inc. bank branch.
Meanwhile, the police commander of Colombia, which Bush will visit on Sunday, said authorities had thwarted leftist rebel plans to disrupt Bush's visit to Bogota. "We have taken measures to neutralize them," said Gen. Jorge Daniel Castro, Colombia's highest-ranking police officer.
Also in Colombia, at Bogota's National University, 200 masked students clashed with 300 anti-riot police and shouted: "Out, Bush." And in Mexico City, about two dozen demonstrators gathered in front of the U.S. Embassy chanting slogans against the U.S. project to construct border fences and Bush's visit.
Reacting to Thursday's demonstrations, White House spokesman Gordon Johndroe said Bush "enjoys traveling to thriving democracies where freeom of speech and expression are the law of the land." Bush himself played down expected protests in interviews ahead of his trip with Latin American news organizations.
"I am proud to be going to a part of the world where people can demonstrate, where people can express their minds," he said in an interview with Univision. And he told CNN En Espanol: "The trip is to remind people that we care."
Chavez, aligned with Cuba's Fidel Castro and a fierce critic of Bush, is marking Bush's trip with a rival tour of the region.
On Saturday, the Venezuelan leader will speak at an "anti-imperialist" rally in a soccer stadium in Buenos Aires, Argentina, about 40 miles across the Plate River from Montevideo, where Bush will be holding talks with Uruguay's president, Tabare Vazquez.
Hadley, Bush's national security adviser, told reporters that instead of worrying about Chavez, Bush was "going to be focusing on those countries and those leaders that have the right model and the right ideas for a better Latin America."
In addition to Brazil, Uruguay and Colombia, Bush is also visiting Guatemala and Mexico.
Bush did not plan visits to any countries that have moved into Chavez's sphere of influence, including Argentina, Bolivia, Ecuador and Nicaragua.
Bush and Brazilian President Luiz Inacio Lula de Silva are expected to announce an "ethanol alliance" on Saturday aimed at creating quality standards for the alternative fuel while joining forces to promote more ethanol use in nations lying between Brazil and the United States.
Silva, in turn, has said he will press the U.S. Congress to repeal or scale back the 54-cent per gallon U.S. tariff on sugar-based Brazilian ethanol. Bush and Silva also were expected to talk about efforts to salvage the World Trade Organization talks - the so-called Doha round - that collapsed in discord last summer over farm subsidies and other disputes.
But he probably can't look to Bush for much help on the tariff issue. White House spokesman Johndroe said tariff matters are "up to Congress" and that Bush wasn't expected to weigh in on the dispute. Among those participating in Thursday's protests were environmentalists and social groups who oppose the biofuels project, fearing that Brazil may clear pristine jungle to ramp up sugarcane cultivation. Greenpeace activists hung a huge banner warning against increased reliance on ethanol as an alternative fuel on a monument to 17th-century Portuguese explorers and conquerors.