Mandela is expected to spend the day privately with his family at their homestead in his southeastern birth village of Qunu. Meanwhile, communities in South Africa and around the world were dedicating 67 minutes of the day to volunteer work and projects for the needy - one minute to mark each of Mandela's 67 years in public service.
Mandela became South Africa's first black president in 1994 after spending 27 years in prison for his fight against racist apartheid rule, and was awarded a Nobel Peace Prize for his efforts.
Tributes to Mandela poured in early Wednesday, with US President Barack Obama saying Mandela "has changed the arc of history, transforming his country, the continent and the world."
Ahmed Kathrada, one of Mandela's oldest friends, said Madiba, as he is affectionately known by his Xhosa clan name, championed the dignity of all.
"You can be rich but if you don't have dignity you are a second-class citizen," Kathrada said in a public lecture marking the birthday celebrations.
Tokyo Sexwale, a longtime ally in the governing African Nation Congress, described Mandela as a global statesman who inspired the world.
At one Johannesburg elementary school on Wednesday, children watched a film documenting Mandela's life and his years of service and sacrifice along with a photographic display of him meeting celebrities including Beyonce, Michael Jackson and Cristiano Ronaldo.
"Nelson Mandela set an example to show us that reconciliation is possible," said 10-year-old Thakgalo Ditabe. She said she wanted Mandela to know how much he meant to her.
Ntando Ntuli, 12, said with pride: "He is my hero because he fought for us. He is an icon, the king of Africa."
In 2009, the United Nations established Nelson Mandela International Day to honor the African leader on his birthday through acts of community service.
In many districts, South Africa came to a virtual standstill early Wednesday as strangers greeted each other in the streets and even infants at one pre-school waved at passersby and sang: "We love you, Tata," or "great father," a supreme term of endearment.
In the eastern port city of Durban Sir Alex Ferguson, manager of England's Manchester United football team that is widely followed in Africa, sang Happy Birthday over a cake iced with the image of the team's yellow and red badge.
Ferguson, who met Mandela on previous visits, said "his presence and personality exudes all around."
Manchester United plays the first game of its South African tour later on Wednesday.
South African churchmen and politicians urged people across the country "to make every day a Mandela Day."
Former US President Bill Clinton got the celebrations off to an early start Tuesday. He and daughter Chelsea met with Mandela in Qunu. Photographs tweeted by one of Mandela's grandsons showed the Nobel Peace Prize winner comfortably seated in an armchair with a blanket over his knees and with the Clintons and his wife, Graca Machel, at his side.
Nobel laureate Archbishop Emeritus Desmond Tutu said the greatest gift the nation could give Mandela on Wednesday would be "to emulate his magnanimity and grace."
"Mr. Mandela taught us to love ourselves, to love one another and to love our country," Tutu said.
Mandela's activism helped bring democracy and freedom to the once white-ruled South Africa. But the country remains beset by tensions over continued white minority domination of the economy, massive unemployment, poor education and health services and the millions who remain homeless or in shacks.