UN Secretary General Kofi Annan has urged governments to endorse "bold and far-reaching" reforms of the body.
Kofi Annan wants world leaders to "act boldly"
They include enlarging the Security Council, setting out rules on when it can authorise military force, and an agreed definition of terrorism.
The proposals are designed to ensure the UN, which was shaken by the bitter debate over the war against Iraq, remains at the heart of world security.
The UN must be brought in line with "today's realities", Mr Annan said.
The reform proposals come at a time when the world body faces criticism over its management of the oil-for-food programme in Iraq and allegations of sexual abuse by UN peacekeepers in the Democratic Republic of Congo.
The reforms proposed will be discussed by a meeting of world leaders attending a UN summit in September, and must then be endorsed by the General Assembly.
Mr Annan said: "This hall has heard enough high-sounding declarations to last us for some decades to come.
"We all know what the problems are and we all know what we have promised to achieve. What is needed now is not more declarations or promises, but action - action to fulfil the promises already made."
In a report setting out the reforms, Mr Annan urges governments to "act boldly" and adopt "the most far-reaching reforms in the history of the United Nations".
"We will not enjoy development without security, we will not enjoy security without development, and we will not enjoy either without respect for human rights," he said.
The report comes two years after the US-led invasion of Iraq, which took place without explicit Security Council authorisation.
Mr Annan said current threats such as civil violence, organised crime, terrorism and weapons of mass destruction, as well as poverty and disease, were interconnected.
"I am profoundly convinced that the threats which face us are of equal concern to us all," he told the General Assembly.
The secretary general called on UN members to agree on a definition of terrorism and to take urgent steps to prevent nuclear, chemical and biological weapons from getting into the hands of terrorists.
The report suggests that "any action constitutes terrorism if it is intended to cause death or serious bodily harm to civilians or non-combatants with the purpose of intimidating a population or compelling a government or an international organisation to do or abstain from doing any act".
Mr Annan also called on developed countries to increase their spending on development and debt relief, to help achieve targets on halving extreme poverty and achieving universal education.
He declared a policy of zero tolerance for sexual exploitation and called for better oversight of UN contracts.
And he proposed a new human rights body and gave his backing to the idea of expanding the 15-member Security Council to 24.
Many of his suggestions are based on the recommendations made last year by a panel that he commissioned.