Former UN Secretary-General Kofi Annan, the first black person to head the organisation, died in the Swiss city of Bern after a short illness.
The Ghanaian career diplomat served as chief of the organisation between 1997 and 2006, having been elected to head the organisation twice.
Annan joined the UN in 1962 after studying management at the Massachusetts Institute of Technology and worked his way up the UN hierarchy.
He eventually rose to become the head of the UN's peacekeeping efforts before assuming the top organisation's top job.
World leaders, past and present, reacted to the news of his death after it was announced on Saturday.
Antonio Guterres - UN secretary-general
"Kofi Annan was a guiding force for good. I join the world in mourning his loss. In these turbulent and trying times, his legacy as a global champion for peace will remain a true inspiration for us all."
Nana Akufo-Addo - Ghana president
"The Government and people of Ghana, First Lady Rebecca and I are deeply saddened by the news of the death, in Berne, Switzerland, of one of our greatest compatriots, Mr Kofi Annan," the Ghanaian leader said, adding that flags at the country's government offices and diplomatic missions would fly at half-staff for a week.
Theresa May - British prime minister
"Sad to hear of the death of Kofi Annan. 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. My thoughts and condolences are with his family."
Vladimir Putin- Russian president
"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."
Angela Merkel - German chancellor
"Kofi Annan never gave up working for the good in the world. [He] knew how to get people engaged, and became a role model, especially for young people all over the world."
Muhammadu Buhari - Nigerian president
"Annan’s humility, nobility and love for humanity set him apart for global greatness, achieving recognition and commendation for the reform of the United Nations’ bureaucracy and multiple interventions to bring peace to the world."
Emmanuel Macron - French president
"We will never forget his calm and resolute approach to matters, nor the strength of his commitments."
Barack Obama - former US president
"Kofi Annan was a diplomat and humanitarian who embodied the mission of the United Nations like few others. His integrity, persistence, optimism, and sense of our common humanity always informed his outreach to the community of nations."
Jean-Claude Juncker - European Commission chief
"Today, the world mourns a great leader and humanitarian but celebrates a life full of courage, empathy and remarkable public service. 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."
Jens Stoltenberg - NATO secretary-general
"The UN and the world have lost one of their giants."
Margot Wallstrom - Swedish foreign minister
"Tragic to learn about Kofi Annan’s death. He personified what is best about the UN, and also had to deal with some of it’s hardest challenges. A great listener, and someone who made everyone want to listen."
Jacinda Ardern - New Zealand prime minister
"The world has lost a great humanitarian, leader, and self proclaimed 'stubborn optimist'. Rest in peace, Kofi Annan."
Narendra Modi - Indian prime minister
"We express our profound sorrow at the passing away of Nobel Laureate and former UNSG Mr Kofi Annan. The world has lost not only a great African diplomat and humanitarian but also a conscience keeper of international peace and security."
Jeremy Corbyn- British opposition leader
"Kofi Annan dedicated his life to building a more just and peaceful world. His efforts in support of conflict resolution and human rights will be remembered. He looked for a peaceful path when others looked for war. My thoughts are with his family and loved ones."
Javad Zarif- Iranian Foreign Ministry
"Extremely saddened by passing of Kofi Annan, a toweing global leader and an unwavering champion for peace, justice and rule of law. Rest in peace my dear old friend."