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 W Bush is expected to face vocal opposition over plans to create a Free Trade Area of the Americas (FTAA).
Venezuela's leader Hugo Chavez, a key opponent, told protesters: "Here, in Mar del Plata, FTAA will be buried!"
Addressing the rally in a football stadium, Mr Chavez called for help to beat the US-backed free trade proposal.
"Only united can we defeat imperialism and bring our people a better life," he said.
Mr Chavez also said US imperialism would fail to halt his left-wing revolution in Venezuela.
'Throw out Bush'
Alongside him stood Argentine former football legend Diego Maradona, wearing a T-shirt accusing Mr Bush of war crimes. "Argentina is dignified. Let's throw out Bush!" he cried.
Earlier, protesters had surrounded a train that brought their comrades from Buenos Aires, among them Bolivian left-wing presidential candidate Evo Morales.
Hugo Chavez and Diego Maradona spoke to cheering protesters
Mr Bush 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, at which the two acknowledged their policy differences.
The US president wants free trade relations to be instituted across the two continents.
Mexican President Vicente Fox has said 29 of the 34 summit nations are 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.
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.
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.
The rivalry between Mr Bush and Mr Chavez is expected to dominate the meeting.
Asked at a news conference how he would approach Mr Chavez, Mr Bush replied that he would be "polite".
The Venezuelan government has said that it will reject any summit declaration which contains references to free trade in the Americas.
The BBC's South America correspondent says Washington still has many strong allies among Latin American countries, who are well aware that the US remains their dominant trade partner.
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.