The summit of two-thirds of the world's nations opened on Friday in Havana with a stream of anti-American rhetoric.
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.
Raul, the acting president of Cuba, said the world today is shaped by Washington's irrational pretensions for world dominance.
"When there no longer is a Cold War, the US 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."
Support for Iran
During the conference, Hugo Chavez, the Venezuelan president, pledged his support for Iran if it is invaded by the US for its nuclear aspirations, and threatened to cut oil supply to the US if Cuba is invaded.
"Iran is under threat; there are plans to invade Iran. Hopefully it won't happen, but we are with you," Chavez told Mahmoud Ahmadinejad, the Iranian president.
Chavez said: "Under any scenario, we are with you just like we are with Cuba, 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."
Ahmadinejad told the assembly on Friday that "Cuba's fight for liberation from imperialism has been a source of inspiration for the world's peoples".
Growth of the assembly
The Nonaligned Movement was formed during the Cold War to establish a neutral third path in a world divided by allegiances to the US 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 UN 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, urging the gathered developing nations to unite against unacceptable acts of aggression.
Raul Castro (R) with Malaysian
PM Abdullah Ahmad Badawi
Raul Castro said: "With regard to international relations, we are not the decisive force that we could be. 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."
For his part, Kofi Annan, the UN secretary-general, 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," he said.
The US declined an invitation to attend, and said it would have no comment on the proceedings.