Cuba assumed the leadership of the Nonaligned Movement on Friday, but with Fidel Castro out of sight, it fell to his younger brother to open the summit of two-thirds of the world's nations with a stream of anti-American rhetoric.

Acting President Raul Castro said the world today is shaped by the United States' "irrational pretentions for world dominance."

"When there no longer is a Cold War, the United States spends one billion dollars a year in weapons and soldiers and it squanders a similar amount in commercial publicity," he said. "To think that a social and economic order that has proven unsustainable could be maintained by force is simply an absurd idea."

CountryWatch: Cuba

Raul Castro acknowledged his older brother's illness after accepting Cuba's three-year chairmanship with a round of applause from the 118-nation group, saying "Comrade Fidel has asked that I transmit to you his most cordial greetings."

But while the iconic leader is under doctors' orders not to preside over the summit, he could still make an appearance, Cuba's foreign minister told the assembly.

"Despite the rigor and will with which he pursues his treatment and physical therapy, the doctors have insisted that he continue to rest," Felipe Perez Roque said, but "his health improves continually and his convalescence is satisfactory."

Many are wondering whether the 80-year-old Castro, a living symbol of revolution for many in the developing world, will be able to guide the group formed during the Cold War in its search for relevance in this era of globalization.

And in the United States, the administration of President George W. Bush has tried to hasten the end of the Castro brothers' rule. On Friday, it proposed that Cubans hold a referendum to decide if they want to be ruled by Raul Castro — a suggestion certain to be rejected by the island's communist leadership. There was no immediate mention of the U.S. proposal at the summit.

Castro temporarily ceded Cuba's leadership to his 75-year-old brother and a handful of other top officials after emergency intestinal surgery in July. And while Cuban officials raise expectations of Fidel's return to power, he's remained out of sight this week, appearing only in photos and video in state media, wearing pajamas in meetings with his close friend Venezuelan President Hugo Chavez and U.N. Secretary-General Kofi Annan.

Chavez, who like Fidel is famous for giving long speeches, spoke well beyond his allotted time, saying that because the ailing Castro wasn't there, he would take that liberty. It was the second such statement in as many days by Chavez, who has moved forcefully to assert himself as an heir to the ailing Cuban leader. "Patria o muerte, vinceremos!" he thundered, echoing Fidel's favorite slogan: "Homeland or Death, We will overcome!"

Chavez also upstaged Raul Castro at a side meeting of the "Group of 15" developing nations while pledging Venezuela's support for Iran if it is invaded because of its nuclear dispute with the U.N. Security Council.

"Iran is under threat; there are plans to invade Iran. Hopefully it won't happen, but we are with you," Chavez told Iranian President Mahmoud Ahmadinejad.

"Under any scenario, we are with you just like we are with Cuba," Chavez told Iran. "If the United States invades Cuba, blood will run ... We will not have our arms crossed while bombs are falling in Havana or they carry Raul off in a plane."

It was not clear what Chavez would do for Iran. He's vowed to cut off Venezuelan oil supplies to the U.S. if it invades Cuba.

Ahmadinejad told the assembly Friday that "Cuba's fight for liberation from imperialism has been a source of inspiration for the world's peoples," and Malaysian Prime Minister Abdullah Ahmad Badawi said the movement "will once again be in Cuba's very capable hands."

The Nonaligned Movement was formed during the Cold War to establish a neutral third path in a world divided by allegiances to the United States and the Soviet Union. It now counts 118 members with the addition of Haiti and St. Kitts this week.

Chavez has been outspoken in advance of next week's U.N. General Assembly session in New York, where nations will debate Iran's nuclear ambitions and Venezuela's campaign for a security council seat.

But Raul Castro also spoke forcefully Friday, urging the gathered developing nations to unite against "unacceptable acts of aggression essentially motivated by the insatiable appetite for strategic recourses."

"With regard to international relations we are not the decisive force that we could be," he said. "The Nonaligned Movement now has to wage courageous battles against unilateralism, double standards, and the impunity granted to those in power, for a fairer and more equal international order."

Annan said the world has changed dramatically since Cuba last hosted the movement in Havana 27 years ago, and that developing nations have new responsibilities and opportunities to promote democracy, protect human rights and develop a civil society.

"The collective mission of this movement is more relevant than ever," Annan said.

The United States declined an invitation to attend, and said it would have no comment on the proceedings.