The Non-Aligned Movement (NAM) summit also issued veiled attacks on the United States and Israel by urging members to close ranks in the face of threats that "have a common origin".
Leaders and high-level representatives from Cuba, Iran, North Korea, Venezuela and Syria are among the countries attending the six day conference.
Cuban Foreign Minister Felipe Perez Roque said in his opening speech that the summit "happens to coincide with the tightening of pressures against Iran for the exercise of its sovereign right to develop a program for the peaceful use of nuclear energy".
Among the prominent leaders slated to attend is Iranian President Mahmoud Ahmadinejad, who has defied UN demands that he halt uranium enrichment, the process used to make nuclear reactor fuel but also atomic bomb material.
The NAM backed Iran in the standoff, with a draft of the summit's final document stressing the right of developing nations to use and produce nuclear energy.
Officials were preparing a declaration on the issue, which would be adopted in addition to the summit's final document, according to a Cuban official, who said the group working on the Iran statement was headed by Indonesia.
At the same time, the draft final document condemned Israel "for continuing to develop and stockpile nuclear arsenals."
The summit will also give nuclear-armed rivals India and Pakistan an opportunity to jumpstart peace talks aimed at resolving their decades-old dispute over Kashmir, a Himalayan region divided between the two countries but claimed in its entirety by both.
"I hope and will make full efforts to make the talks substantive, so that these are result-oriented," said Pakistini President Pervez Musharraf, who was scheduled to meet Indian Prime Minister Manmohan Singh on the sidelines of the summit.
The bilateral talks would be the first high-level contact between the two countries since attacks on commuter trains in Mumbai in July stalled the peace process. New Delhi had pointed the finger at Islamabad and a Pakistan-backed Islamic rebel group for the blasts which killed 183 people and wounded more than 800.
Cuba and several other members of the NAM have stressed the need to give new impetus and focus to the movement, created during the Cold War to counter the hegemony of the superpowers. Now, they say, they must work against overwhelming US might.
On the first day of the gathering, it remained unclear if Cuban President Fidel Castro, 80 years old and convalescing after surgery, would be well enough to show up as scheduled, though he has said he would be meeting some of the visiting dignitaries.
The gathering should mark the international debut by Raul Castro, 75, who is officially in charge of Cuba until his more prominent brother is well enough to get back to work full time.
Cuba will take over the leadership of the NAM from Malaysia. Dissidents said this would be a good time for the communist government to adopt democratic reforms.
"It's the perfect occasion for the Cuban government to ... allow free and public expression and ... respect human rights," the Progressive Arch dissident group said.
The summit will also form the backdrop for rival lobbying from Venezuela and Guatemala for one of the 10 temporary seats on the 15-member UN Security Council, ahead of elections at UN headquarters in coming weeks.
Venezuela's staunchly anti-US President Hugo Chavez recently conducted a 10-day tour of Asia and Africa that earned the oil-rich South American country support for its UN bid as well as trade deals.
The heads of state and government were slated to start their talks on Friday, after four days of preparatory meetings. Haiti and St Kitts and Nevis were set to be formally accepted into the movement, swelling its ranks to 118.