"One of the great honors of my life was to be invited to Nelson Mandela's home, spend private time and get to know him," Oprah Winfrey Tweeted, posting a photo on Instagram of their time together.
"He was everything you've ever heard and more – humble and unscathed by bitterness. And he always loved to tell a good joke," Oprah said. "Being in his presence was like sitting with grace and majesty at the same time. He will always be my hero. His life was a gift to us all."
Human-rights advocate Martin Luther King III, in a statement, also spoke out to pay tribute to Mandela's invaluable impact on social justice.
"He was a symbol of dignity for people of African descent and for the poor and oppressed everywhere," King said.
"Through his and his people's long walk to freedom, Mr. Mandela's constant fight for equality personified, what my father often said, 'Injustice anywhere is a threat to justice everywhere.' "
A Mandela SchoolIn New York City, outgoing Mayor Michael Bloomberg announced Friday that the city would create a new high school to honor Mandela, and it'll be housed in the same Brooklyn Boys & Girls Club that the leader visited in 1990, the New York Post reported.
Bloomberg said such a school was the perfect legacy for Mandela, who viewed education as transformational.
"I think a school named after him and at a location he visited is a nice way to pay tribute," Bloomberg said. "The amazing thing is how he brought people together. After that number of years in jail, you'd think he'd come out with a score to settle, and he was much smarter than that."
The Elders MournThe Elders, a group of global leaders that was founded by Mandela in 2007, offered their own tribute on their website, noting they were "deeply saddened" by his death. They posted a tribute video honoring his life.
"Mandela – or Madiba as he is known in South Africa – called the Elders together in 2007, urging them to be bold, independent and to speak the truth. He told them to be a robust force for good, and to work in the interests of peace for all humanity," they wrote.
"The Elders have taken Madiba's words as their mission and endeavor to honor his memory in their work. On this sad day they give their love and condolences to his wife Graça Machel, who is also a member of The Elders, and all the Mandela family."
The group's chairman, Kofi Annan, former secretary-general of the United Nations, added his own statement of support.
"The world has lost a visionary leader, a courageous voice for justice, and a clear moral compass. By showing us that the path to freedom and human dignity lies in love, wisdom and compassion for one another, Nelson Mandela stands as an inspiration to us all," Annan wrote.
"I shall never forget his expansive smile and gentle demeanor, nor his steely determination and wonderful sense of humor. I have lost a dear friend," said Annan. "While I mourn the loss of one of Africa's most distinguished leaders, Madiba's legacy beckons us to follow his example to strive for human rights, reconciliation and justice for all."
Symbols of GriefAt the White House, photographer Pete Souza Tweeted a photo of a somber President Obama, watching ongoing news coverage of Mandela's death. Obama ordered flags to be lowered half-staff until sunset Monday at the White House and other military and public facilities around the country in tribute.
The United Nations has also flown its flag at half mast in honor of the political leader, while the Eiffel Tower has been lit up in the colors of the South African flag.