The Sixth Summit of the Americas formally opened under tight security in this Colombian Caribbean resort city with US President Barack Obama and most other democratically elected leaders of the Western Hemisphere in attendance.
Venezuelan President Hugo Chavez, who is suffering from cancer, did not make the trip on the advice of his doctors. He was to travel to Cuba for further radiation therapy to treat a recurrence of cancer, according to his foreign minister Nicolas Maduro.
Also absent was Nicaraguan President Daniel Ortega, Honduran President Porfirio Lobo said. No reason was immediately given.
Ecuadoran President Rafael Correa decided to boycott the summit because of the exclusion of Cuba, one of its allies and the Americas' only one-party Communist state. The United States says Cuba is ineligible due to what US officials say is a lack of democracy in Cuba.
The official theme of the gathering is "Connecting the Americas: Partners for Prosperity," and Obama hailed the rapid progress across the region.
"It is remarkable to see the changes that have been taking place in a relatively short period of time in Latin, Central America and in the Caribbean," Obama said.
"We've seen enormous progress. Trade between the United States and Latin, Central America and the Caribbean has expanded 46 percent since I came to office."
But most Latin American leaders were mainly preoccupied by two issues which officially are not the agenda: the pros and cons of drug legalization, and Cuba's continued exclusion from the summit.
The devastating effects of drug trafficking is of particular concern to Central American leaders.
Several of them met here on the sidelines of the summit to discuss Guatemala's proposal to consider legalizing street drug consumption. But they failed to reach consensus.
The presidents of Nicaragua and El Salvador, Daniel Ortega and Mauricio Funes, did not attend.
Guatemalan President Otto Perez insisted that his idea remained very much alive and voiced hope it would be taken up at a private meeting of hemispheric leaders.
Speaking at a summit of regional corporate leaders ahead of the summit, Obama said he favored a debate on the issue. On Friday, he said he opposed decriminalization or legalization of drugs.
"I think it is a valuable agenda to have a conversation whether the laws in place are laws that are doing more harm than good in certain places," the US leader said Saturday. "we can't look at the issue of supply without looking at the issue of demand coming from the United States."
On Cuba, Obama said Friday that the communist regime in Havana lacks the democratic credentials to attend the summit.
But summit host, President Juan Manuel Santos of Colombia, said Saturday it would be "unacceptable" to keep Cuba out of the next summit.
"The isolation, the indifference has shown its ineffectiveness. In today's world, there is no justification for this anachronism," he added.
Cuba has never taken part in a Summit of the Americas. And in early March, Santos traveled to the Cuban capital to explain that a lack of consensus had prevented Cuba from being invited this time.
The United States and Canada are opposing any reference to Cuba in the summit's draft final statement prepared by the region's foreign ministers, according to diplomats.
Argentine Foreign Minister Hector Timerman meanwhile has been pushing the summit to issue a "new show of solidarity" with Buenos Aires' claim to the British-ruled Falkland islands.
Argentina and Britain this month marked 30 years since they went to war over the remote South Atlantic islands.
But the draft final statement prepared by Colombia focuses on poverty eradication, regional integration, transnational crime and increased access to technology, leaving aside controversial issues such as Cuba, the drug fight and Argentina's claim on the Falklands.