- In 2001, Mr. Annan won the Nobel peace prize for his reforms in the United Nations as well as giving priority to human rights.
"Kofi was humanity's best example, the epitome, of human decency and grace".
The UN said it would fly flags at half mast at all of its locations around the world through Tuesday.
Ghanaian President Nana Akufo-Addo announced a week of national mourning, calling Annan "one of our greatest compatriots".
Annan, one of the world's most celebrated diplomats and a charismatic symbol of the United Nations, started at the United Nations in 1962 as a low-ranking officer with the World Health Organization in Geneva.
"Honestly speaking, we are in a mess", the Nobel peace laureate said on December 12 ahead of a major climate conference in Paris.
Key proposals included the introduction of strategic management to strengthen unity of objective, the establishment of the position of Deputy Secretary-General, a 10-percent reduction in posts, a reduction in administrative costs, the consolidation of the United Nations at the country level, and reaching out to civil society and the private sector as partners.
He told the BBC in September 2004 that the Iraq war did not conform to the United Nations charter and was illegal.
Annan later admitted that the scandal had sorely tested his mettle not only as secretary-general, but as a father.
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Democratic Alliance MP Stevens Mokgalapa described Annan as "one of the great African diplomats of our time" and a "global statesman".
Post-election violence had broken out after Kenya's former president Mwai Kibaki was declared victor of the 2007 presidential polls against opposition leader Raila Odinga.
A year later, his reputation was boosted after he successfully helped negotiate a power-sharing deal to end post-election violence in Kenya.
"He played a vital role in leading The Elders' work, and was a voice of great authority and wisdom in public and private".
The Elders, a group of former leaders including Gro Harlem Brundtland and Mary Robinson, paid tribute to their inspiring chairman, noting his visits to South Africa and Zimbabwe in July. After his death was announced, leaders around the world remembered his impact on global politics.
"Grandpa used to tell us a lot about how he was such a nice person", said Kojo Manu, a mechanic who said his late grandfather had lived in the same neighborhood with Annan.
Kofi Annan led the world in to the new millennium, making a big impression on the diplomatic stage, although he didn't always agree with the actions of the most powerful countries, calling the 2003 invasion of Iraq "illegal".
"He was a great African by every standard", lawmaker Ras Mubarak said.
North Atlantic Treaty Organisation chief Jens Stoltenberg said "the United Nations and the world have lost one of their giants".