UN secretary general Ban Ki-moon says Nelson Mandela was "a giant for justice" whose "selfless struggle for human dignity, equality and freedom" inspired many people around the world.
"No one did more in our time to advance the values and aspirations of the United Nations," Ban told reporters soon after Mandela's death was announced on Thursday.
Ban said he was deeply touched when he met Mandela at his residence in South Africa in February 2009.
"When I thanked him for his life's work, he insisted the credit belonged to others," Ban said. "I will never forget his selflessness and deep sense of shared purpose."
The UN Security Council interrupted a meeting on the tribunals for former Yugoslavia and Rwanda and stood for a minute in silent tribute to Mandela.
Argentina's UN ambassador Maria Cristina Perceval, who had been speaking, called Mandela "a man who gave hope to the entire world."
"Good men and women, men such as Mandela resisted and taught us to resist fear ... to resist oblivion," she said.
The Security Council said in a statement later that "Mandela will forever be remembered as someone who gave up so much of his life in the struggle for freedom, so that millions could have a brighter future."
The UN General Assembly in 2009 adopted a resolution declaring Nelson Mandela International Day, the first ever international day in honor of an individual. It is celebrated every year on July 18, his birthday.
"The members of the Security Council consider this to be a reflection of the magnitude of Nelson Mandela's contribution to freedom and justice," the council said. "Nelson Mandela day is a celebration of the idea that each individual has the power to transform the world, and the ability to make an impact, just as Nelson Mandela did himself."