More than 1000 rioters clashed with police, setting bonfires in the streets and destroying shopfronts across about six square blocks near the site of the fourth Summit of the Americas.
Aljazeera's correspondent said police clashed with protesters who tried to vandalise shops selling American products.
A group of demonstrators wearing bandanas over their faces and beating wooden clubs against the pavement faced off with riot police, who responded by firing tear gas.
Car sirens wailed as frightened pedestrians fled. Police held fast behind the barricade and prevented what appeared to be an attempt by the demonstrators to break through.
Demonstrators then set American flags on fire, while others used slingshots to fire rocks at police. Several shops, including a mini-market and a pastry store, had their windows shattered during the rioting.
The violence came after a massive, peaceful march by about 10,000 demonstrators earlier in the day who listened to Venezuelan President Hugo Chavez urge them to fight the US policies of President George Bush, including a proposal to create a hemisphere-wide Free Trade Agreement (FTAA).
Aljazeera's correspondent in Mar Del Plata said hundreds of demonstrators carried banners condemning the invasion of Iraq and presence of US and UK troops there.
Earlier, Chavez spoke to a cheering crowd at a cavernous basketball stadium, urging protesters to help him "bury" the FTAA.
Some demonstrators condemned
the US military presence in Iraq
The planet "is being destroyed under our own noses by the capitalist model, the destructive engine of development," Chavez said, adding that "every day there is more hunger, more misery thanks to the neoliberal, capitalist model".
Supporters of free trade say those policies are not to blame for the region's problems and argue that the pact would promote growth and boost the region's economies.
Chavez also pledged a "100-year" war if the United States invades Venezuela - something he has repeatedly accused the American government of planning.
US officials deny any such plans.
The protest also featured Argentine soccer great Diego Maradona and Bolivian presidential hopeful Evo Morales, who both travelled to Mar del Plata aboard an overnight train packed with protesters chanting "Fascist Bush! You are the terrorist!"
Smoking cigars given to him by Cuban President Fidel Castro, Maradona said: "We are going to stand against the human trash known as Bush."
Maradona has been angry at the United States since being denied a visa to a drug treatment centre in Miami in the 1990s.
Morales called on the Latin American people to unite to "oppose the politics of hunger and destroy Bush".
The summit also sparked demonstrations in Venezuela and in the Argentine capital of Buenos Aires, where government employees threw anti-Bush fliers from their office windows.
Chavez urged South Americans
to 'bury' the FTAA
In Caracas, thousands of Venezuelans marched to support Chavez's stance against the Free Trade Area of the Americas. People in the march, which was organised by Chavez's party, shouted: "Long live Chavez!" and "Bush go home!"
Journalists estimated the crowd at 3000 to 5000 marchers. Police did not give any estimate.
Marcher Joel Capriles, 44, carried an effigy with the face of Bush and a message saying "hit the terrorist," inviting people to take swipes at it.
Bush "betrayed the will of the people of Latin America," Capriles said, referring to US efforts to push for a hemisphere-wide free-trade zone.