Thousands of protesters chanting "Get out Bush" have thronged the streets of Mar del Plata, an Argentine beach town hosting the Summit of the Americas.
The US president and 33 other regional leaders are in town to discuss free
trade and poverty, amid tight security.
George Bush is expected to face vocal opposition, not just from left-wing protesters, but from other leaders like Venezuela's Hugo Chavez.
Mr Chavez was due to address the protesters before joining the summit.
The crowd marched through the streets to a football stadium.
Shops were boarded up along the route, though the march was mainly peaceful.
Earlier protesters had surrounded a train that brought their comrades from Buenos Aires, including protest leaders like Argentine former football legend Diego Maradona and Bolivian left-wing presidential candidate Evo Morales.
Rivalry
Mr Bush arrived on Thursday and held a meeting on Friday morning with leaders
from the Central American Free Trade Area.
He also appeared at a news conference alongside his host, Argentine President Nestor Kirchner.
The US president wants free trade relations to be instituted across the two
continents.
In his keynote speech, Mr Bush will argue that the way to guarantee prosperity
is by encouraging free trade and a flourishing private sector and by deepening
democracy, the BBC's Jamie Coomarasamy in Mar del Plata reports.
But emotions are high among those who say US-backed free-market policies have pushed millions into poverty.
Some 96 million people in the region are surviving on less than $1 per day, according to the United Nations.
The rivalry between Mr Bush and Mr Chavez is expected to dominate the meeting.
The Venezuelan government has said that it will reject any summit declaration which contains references to free trade in the Americas.
Strong allies
However, Mexican President Vicente Fox announced a majority of the 34 summit
nations were willing to move forward with free trade negotiations without
dissenting countries.
Apart from Venezuela, those nations opposed to the creation of a huge free trade zone include Argentina, Brazil, Uruguay and Paraguay.
More than 8,000 police officers are guarding the venue of the summit.
"We hope protests are carried out in a peaceful way, but if they are
not, we are prepared to give wrongdoers a forceful response," said federal
police commissioner Daniel Rodriguez.
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