Hundreds of protesters have run riot in Argentina, throwing stones at police just blocks away from the opening of a summit attended by 34 Americas leaders.
Groups of demonstrators approached security cordons around the summit, and a bank was set on fire as police fired tear gas to disperse the rioters.
US President George W Bush is one of the leaders present, for discussions which include free trade and poverty.
Mr Bush faces opposition over plans to revive talks on a free trade area.
As the meeting opened, there was no indication that any compromise had been reached.
Venezuela's leader Hugo Chavez told protesters earlier he would bury the Free Trade Area of the Americas (FTAA) - and defeat US "imperialism".
More than 8,000 police have been drafted in to protect the fourth Americas summit, being held in the Argentine beach town of Mar del Plata.
Protests began peacefully, as thousands of demonstrators chanted "Get out Bush".
They marched through boarded-up streets a few kilometres away from the summit venue itself.
Demonstrators included Argentine former football legend Diego Maradona.
But smaller groups armed with wooden clubs and wearing bandanas began burning US flags, throwing stones and petrol bombs, smashing windows and setting fire to shops.
Police used tear gas to disperse them, slowly pushing them away from the summit area down one of the town's main streets.
Local media reports said 20 demonstrators had been injured in clashes with police.
Police said 64 people were arrested, and the situation was "under control".
Protesters argue that US-backed free-market policies have pushed millions into poverty in the region.
Addressing their rally in a football stadium, Mr Chavez said: "Here, in Mar del Plata, FTAA will be buried!"
He called for help, saying: "Only united can we defeat imperialism and bring our people a better life."
Other demonstrations were held in the Argentine capital Buenos Aires, and in Uruguay and Venezuela.
Diego Maradona and Hugo Chavez criticised US policies
The Venezuelan government has said that it will reject any summit declaration which contains references to free trade in the Americas.
However, Mexico's 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.
US interest renewed
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 BBC's South America correspondent Steve Kingstone says Mr Chavez's growing influence helps explain Washington's renewed involvement in the region.
Security, democracy and trade are the main areas of interest for the US in Latin America, and on all three counts, Venezuela's leader is causing concern, our correspondent says.
Washington is concerned about military build-up in Venezuela, suspects it of meddling in Bolivia's election campaign and fears Mr Chavez's talk of closer ties between Latin American nations may attract leaders away from US plans for a huge free trade zone, he adds.