Leaders from the nine member Alba bloc are at the conference
Cuba's President Raul Castro and Venezuelan leader Hugo Chavez have criticised the US during a key alternative trade summit in Havana.
Mr Castro accused the US of treating Latin America as its "backyard" and denounced a deal giving US armed forces access to Colombian military bases.
Meanwhile Mr Chavez said US criticism about Latin America's relationship with Iran was an "imperial offensive".
Both were speaking at the opening of the left-wing Alba summit in Havana.
A regional political and economic bloc, the Bolivarian Alliance for the Peoples of America (Alba), was founded five years ago as a radical alternative to America's free trade policies in the region.
The BBC's correspondent in Havana Michael Voss says Mr Castro's harsh rhetoric was reminiscent of the Cold War.
It could be a signal that ties between Cuba and the US might be taking a turn for the worse.
Raul Castro and Hugo Chavez both criticised the US
Mr Castro said: "On the one side, there's an exploitative model of dependence subordinated to the interests of the empire, on the other, the advance of revolutionary and progressive forces, representing the disposed."
Mr Castro said President Obama had carried out a hegemonic offensive with the Colombia deal.
He also criticised Washington for presiding over what he described as "an electoral farce" in Honduras.
He said the country's people "have been deprived of their constitutional rights and there has been imposed, with the support of the US administration and a coup-led government (in Honduras), an electoral farce".
Shortly afterwards Mr Chavez openly rejected a warning made by US Secretary of State Hillary Clinton on Friday that Latin American countries needed to "think twice" about fostering a relationship with Iran.
Iran's President Mahmoud Ahmadinejad last month visited Brazil, Venezuela and Bolivia which voiced support for Iran's right to a nuclear programme.
But the US is concerned about what the consequences of the stepped up diplomatic efforts could mean.