Clashes broke out in Brazil's largest city as US President Bush arrived at the start of a six-day regional tour.
At least 20 people were injured in clashes with riot police in Sao Paulo after thousands turned out to protest against George W Bush's visit.
The US leader is due to meet Brazil's Luis Inacio Lula da Silva to sign an ethanol energy alliance.
He will also go to Uruguay, Colombia, Guatemala and Mexico on a tour aimed at strengthening regional ties.
In Sao Paulo, about 10,000 people spilled out along one of the city's broadest avenues, in the heart of the financial district, banging drums, waving red flags and carrying banners reading "Bush Go Home".
Although largely peaceful, clashes flared between some of the protesters and police.
Demonstrators threw rocks and sticks at some of the 4,000 police patrolling the streets.
Riot police responded by firing tear gas and lashing out with their batons.
Sixteen police officers were reported to be among those injured.
Many of the demonstrators are angry at the war in Iraq and the proposed ethanol deal, which they say is an attempt to control the country's production of the bio-fuel which powers eight out of 10 new cars in Brazil.
Together with the US, Brazil produces about 70% of the world's ethanol, a bio-fuel made from sugar cane or corn.
The proposed accord would make ethanol an internationally traded commodity and promote its production in Central America and the Caribbean.
But activists say sugar cane cultivation is water intensive and responsible for stripping the Amazon rainforest.
They also say the ethanol production process is concentrated in the hands of a few powerful families or corporations.
Ahead of Mr Bush's arrival, hundreds of protesters occupied the Cevasa sugar and ethanol-processing plant in Sao Paulo state that was recently sold to a US company.
Correspondents say Washington is adjusting to the election of increasing numbers of left-wing administrations in Latin America, including some that are openly critical of the US.
Coinciding with Mr Bush's visit to Uruguay, Venezuelan President Hugo Chavez will make an appearance on Friday in Argentina.
Mr Chavez is expected to hurl insults at Mr Bush at a rally in a football stadium in Buenos Aires.
The BBC's Daniel Schweimler, in Buenos Aires, says Mr Chavez will find an enthusiastic reception to his fiery brand of anti-US rhetoric.