Mandela was admitted to a hospital in Pretoria for a recurring lung infection on June 8, causing widespread concern he was on his deathbed.
Thursday's activity seemed to indicate an improvement in Mandela's health, though he remained very fragile. Details of his medical condition were kept tightly controlled by his family and current South African President Jacob Zuma.
Mandela is making "remarkable progress," said one of his daughters, Zindzi, after tense weeks.
Children sang "Happy Birthday" at school assemblies nationwide, and many honored the man known as "the father of the nation" by performing acts of charity for 67 minutes, symbolizing Mandela's 67 years of public service. World leaders praised the anti-apartheid leader's life of sacrifice and vision.
At the United Nations, where July 18 was declared Nelson Mandela International Day, former President Bill Clinton recalled getting to know him while in office. "His heart was so big, and his humanity so great that we often had trouble keeping our official roles apart from our personal friendship," Clinton said.
Also known by his clan name Madiba, Mandela was jailed for 27 years under apartheid and led a difficult transition to democracy, becoming president in all-race elections in 1994. He served one five-year term, evolving into a global statesman and pursuing charitable causes after that. He retired from public life years ago.
"South Africa is a better place today than it was in 1994 and this is because of the contribution made by Madiba and his collective," the ruling African National Congress, once led by Mandela, said in a statement.
The Associated Press contributed to this report.