World mourns loss of Kofi Annan
He was also commemorated for his strong effort and plan during his time in 2014 on the Cyprus issue.
Current UN chief Antonio Guterres voiced deep sadness at the news, describing his predecessor as "a guiding force for good".
"In many ways, Kofi Annan was the United Nations. He rose through the ranks to lead the organization into the new millennium with matchless dignity and determination. Like so many, I was proud to call Kofi Annan a good friend and mentor."
The UN high commissioner for human rights Zeid Ra'ad Al Hussein said he was grief-stricken.
"Kofi was humanity's best example, the epitome, of human decency and grace. In a world now filled with leaders who are anything but that, our loss, the world's loss becomes even more painful," he said.
"He was a friend to thousands and a leader of millions. There are some human beings who will seem irreplaceable to us, rare human beings. Kofi Annan is high among them. Goodbye my dear friend... goodbye Kofi."
President Nana Akufo-Addo declared a week of mourning, saying the national flag will fly at half-mast at home and in the country's diplomatic missions around the world in honor of "one of our greatest compatriots".
"He devoted his life to making the world a more peaceful and united place. He fought to end suffering and injustices across the world and helped to rebuild bridges where they had been destroyed."
President Vladimir Putin said he was "fortunate" to have been in personal contact with Annan while he lead the UN.
"I sincerely admired his wisdom and courage, his ability to make informed decisions even in the most complex, critical situations. His memory will live forever in the hearts of Russians."
NATO chief Jens Stoltenberg said "the UN and the world have lost one of their giants".
"His warmth should never be mistaken for weakness. Annan showed that one can be a great humanitarian and a strong leader at the same time," Stoltenberg wrote on Twitter.
"A great leader and reformer of the UN, he made a huge contribution to making the world he has left a better place than the one he was born into," British Prime Minister Theresa May said on Twitter.
Former prime minister Tony Blair described him as "good friend".
"We will never forget his calm and resolute approach, nor the strength of his fighting spirit," President Emmanuel Macron tweeted.
Both key figures in the deadly turmoil that followed Kenya's 2008 election paid tribute to Annan, who helped mediate an end to the violence.
Former president Mwai Kibaki said Annan "will be remembered for mediating for the return of peace in Kenya".
Opposition leader Raila Odinga hailed "what became known as 'the Annan Doctrine' in which he made it clear that the need to respect sovereignty cannot be used as a shield by governments to brutalise their own citizens and that the international community has a right to intervene".
President Cyril Ramaphosa praised Annan as a "great leader and diplomat extraordinaire" who had advanced the African agenda within the UN and had "flown the flag for peace" around the world.
The Elders organization -- a group of statesmen co-founded by Annan which speaks out on global issues - hailed him as "a voice of great authority and wisdom in public and private".
"The world has lost an inspiring figure - but one whose achievements will never be forgotten, and whose commitment to peace and justice will endure to inspire future generations," deputy chair Gro Harlem Brundtland said in a statement.