In South Africa, citizens performed at least 67 minutes of public service on his birthday, a reference to the number of years he devoted to helping others.
In Mandela's childhood village of Qunu, relatives, including children and grandchildren, gathered to mark his birthday and perform community service at the local health centres.
Mandela was born on July 18, 1918 in a Thembu royal family in Mvezo village southeastern South Africa.
His family has organised a get together for a meal that will include a champagne toast -- which the ailing Mandela cannot partake in -- and his favorite dish of tripe, a meat delicacy.
In addition to public service, South Africans held various activities to honour a man credited with helping bring together a nation deeply divided by apartheid.
About 14 million students sang "Happy Birthday" in unison nationwide before they started their lessons.
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World leaders also paid tribute to his legacy.
US President Barack Obama and his wife, Michelle, described him as committed to democracy, saying his accomplishments changed South Africa and the world.
"Nelson Mandela's personal story is one of unbreakable will, unwavering integrity, and abiding humility," Obama said in a statement.
"... By any measure, Nelson Mandela has changed the arc of history, transforming his country, continent and the world," he added.
"You can be rich but if you don't have dignity you are a second-class citizen," Ahmed Kathrada, one of Mandela's oldest friends said in a public lecture marking the birthday celebrations.