UN Secretary General Antonio Guterres has called for a sweeping reform of global governance structures.
Today's structures "reflect yesterday's world", Mr Guterres told the Brics summit in Johannesburg on Thursday.
He said, for multilateral institutions such as the International Monetary Fund and World Bank Group to be universal, they must be rethought.
Global governance structures "were largely created in the aftermath of the Second World War, when many African countries were still ruled by colonial powers and were not even at the table", Mr Guterres said on the final day of the summit.
"This is particularly true of the Security Council of the United Nations and the Bretton Woods institutions," he said of the World Bank and IMF.
The UN chief called for "a strengthened and reformed multilateral architecture based on the UN charter and international law".
"Fragmentation is inevitable if multilateral institutions do not reflect today’s power and economic realities," he said.
“In a fracturing world overwhelmed by crises, there is simply no alternative to co-operation.
"Redesigning today's outdated, dysfunctional and unfair global financial architecture is necessary, but it won't happen overnight."
At the summit, the UAE, Saudi Arabia, Egypt, Iran, Ethiopia and Argentina were invited to join the bloc.
Debate over expanding membership beyond current members Brazil, Russia, India, China and South Africa topped the agenda.
"Brics has embarked on a new chapter in its effort to build a world that is fair, a world that is just, a world that is also inclusive and prosperous," said South African President Cyril Ramaphosa at the summit's final day.
Mr Guterres also told the three-day summit that world leaders must drastically step-up climate action and climate justice.