U.S. President George W. Bush arrived in Brazil on Thursday as a massivemarch against his visitto the country's largest city ledto violent clashes betweenprotestersandpolice.

Bush is due to meet Brazilian PresidentLuis Inacio Lula da Silva on Fridaytopush foran ethanol energy alliance on the first stop of his week-longLatin American tour.

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U.S. President George W. Bush and his wife Laura arrive at the air force base in Sao Paulo on Thursday. ((Silvia Izquierdo/Associated Press))

Brazil is mounting what has been described as its biggest security effort ever in Sao Paulo. About 4,000 agents — including Brazilian troops, and FBI and U.S. Secret Service officers — will be on hand during Bush's almost 24-hour visit.

Riot police fired tear gas at protesters and beat them with batons after about 10,000 people held a largely peaceful march through the financial heart of the city.

Hundreds of demonstrators fled and ducked into businesses to avoid the ensuingmayhem on the streets.

Authorities did not immediately report any injuries, but Brazilian media said at least six people were hurt and photographers took pictures of 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.

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Activists carry a banner that, in English, reads "Get Out Bush" to protest the U.S. president's Sao Paulo visit to discuss an ethanol energy alliance. ((Andre Penner/Associated Press))

After the clash, the protest continued peacefully but with far fewer people. The marchers waved communist flags and railed against Bush, the war in Iraq and the ethanol proposal. Almost all had departed by sundown,before the president arrived in Sao Paulo.

Protesting students also lobbed rocks and homemade explosives called potato bombs at riot police on a university campus in the Colombian capital of Bogota, where Bush is scheduled to visit Sunday as part of his five-country tour of Latin America.

The Colombian demonstrators called for the scuttling of a U.S.-Colombia free-trade agreement signed in November and currently stalled in U.S. Congress.

Bush has spoken approvingly of Brazil's ethanol program, which powers eight out of every 10 new cars.

The proposed accord is meant to help turn ethanol into an internationally traded commodity and promote sugar cane-based ethanol production in Central America and the Caribbean.

With files from the Associated Press