Venezuelan President Hugo Chavez is set to attend the Summit of the Americas, but will also speak Friday at the opposition Summit of the People.
Tens of thousands of people are expected to participate in the opposition rally, which has scheduled a march Friday to protest President Bush's presence in Argentina. Some activists say they are participating because they oppose the Iraq war and President Bush's proposed Free Trade Area of the Americas.
Venezuelan ally Cuba is sending a delegation to the opposition summit. Cuba was not allowed to participate in the Summit of the Americas because it is a communist country.
Argentine soccer hero Diego Maradona has also announced he will participate in the protests.
As many as 8,000 security officers have been deployed around the summit site of Mar del Plata in case violence occurs.
Some information for this report provided by AP, Bloomberg and Reuters.