He said: "The Security Council must reform - for the sake of the developing world, and for the sake of the United Nations itself.
"The perception of a narrow power-base risks leading to an erosion of the UN's authority and legitimacy - even, some would argue, its neutrality and independence. I have in the past described this as a democracy deficit."
With Fidel Castro out of sight, his brother Raul presided over the meeting of two-thirds of the world's nations, rallying some of the most outspoken US foes with a speech blaming America for much of the planet's woes.
He said: "The United States spends one billion dollars a year in weapons and soldiers.
"To think that a social and economic order that has proven unsustainable could be maintained by force is simply an absurd idea."
US veto power
North Korea used the summit to defend its nuclear weapons programme, Sudan's leader rejected a UN peacekeeping mission in Darfur, and Iran's president insisted on his nation's right to enrich uranium for nuclear energy.
But the top priority seemed to be balancing US veto power on the UN Security Council.
Mahmoud Ahmadinejad, the Iranian president, said: "The US is turning the Security Council into a base for imposing its politics.
"Why should people live under the nuclear threat of the US?"
Similar sentiments were expressed by Venezuela, Zimbabwe, Belarus and many others.
Kim Yong Nam, North Korea's leader of parliament, said his country "would not need even a single nuclear weapon if there no longer existed a US threat".
Kim added that US insistence on financial sanctions has "driven the situation into an unpredictable phase".