Developing countries attacked the world's richest nations for failing to fulfil commitments to increase financial aid.
Members of the Group of 77 said developed countries were responsible for challenges like global warming that the poorer nations were now grappling with.
The group called for better co-operation among the developing nations and joint action on global challenges such as climate change, food security and poverty to promote their own interests.
Egyptian foreign minister Ahmed Aboul Gheit complained that developing countries were not getting enough aid and did not have the economic clout they deserved.
The "core of the problem" he said, was the "lack of adequate funding as a result of the failure of many developed countries to fulfil their financial commitments".
The Group of 77, which unites 132 mainly developing countries - most of them in the Southern Hemisphere - and China, held a day-long meeting at the United Nations' headquarters in New York, where the chairmanship was handed from Yemen to Argentina.
Undersecretary general Sha Zukang, who oversees economic and social affairs, pointed to the accelerating degradation of the environment and continuing lack of consensus in the international community on how to deal with climate change.
"Against this backdrop of the multiple crises ... the leadership role of the Group of 77 has never become more important," he said.
General Assembly president Joseph Deis said the G77 represented almost two-thirds of the UN member states and was "increasingly being recognised as emerging economic powers".
He called for global action to address the challenges and threats facing a more interdependent world, including the financial crisis and climate change.
Yemeni foreign minister Abubakr al-Qirbi, the outgoing G77 president, said climate change was an urgent challenge for the group because it "threatens not only our societies' developmental prospects but also their very existence".
He said co-operation among developing countries had increased "in importance and scope", but complimented rather than substituted co-operation with developed countries.