Mesic said that although imperfect, the United Nations was irreplaceable and that it had to be enabled to reflect the modern world and cope with the problems of that world.
Mesic said the world could be only partially satisfied with the UN 60 years after its establishment because the organisation had partly failed in fulfilling its basic principles, as well as in achieving the goal of helping in the development of undeveloped countries and preventing wars and terrorism.
"We have to reform the United Nations," Mesic said, addressing heads of state and government at the three-day summit which ends on Friday.
Stressing that Croatia was a successor to one of the countries which founded the UN, Mesic said that Croatia cared much about not betraying the high ideals which governed those who laid its foundations.
Mesic said that the UN reform had to meet some criteria, such as start from the current reality, take into account current development trends, and be acceptable to the convincing majority of the UN countries.
That majority is against war, poverty and terrorism, and for security, development, equality and respect for differences, he said.
Insisting on the unavoidable role of the UN in modern world, Mesic said that the UN must never become a cover giving legitimacy to any policy that would be contrary to the attitude of the majority.
Mesic welcomed the Declaration on the Millennium Development Goals, saying that although it was not what it was originally intended to be, the Declaration still bore witness to the prevailing attitude in favour of goals such as the fight against poverty and child mortality and support for the achievement of general education for all children until 2015.