Millions of South African schoolchildren have raised their voices in song to celebrate Nelson Mandela's 93rd birthday, leading a worldwide chorus of birthday wishes for him.
More than 12 million students sang a special version of Happy Birthday before lessons began on Monday.
Mandela also received birthday wishes from South African leaders and from Barack Obama. In a statement, the US president called Mandela "a beacon for the global community, and for all who work for democracy, justice and reconciliation".
Mandela, who has retired from public life, is expected to spend the day with his family in his home village of Qunu, approximately 600 miles (965km) south of Johannesburg.
People around the world have been asked to mark the occasion by giving 67 minutes of their time to work in their local community – one minute for every year of Mandela's public service. He became South Africa's first black president after spending 27 years in prison for his fight against apartheid. He was awarded a Nobel peace prize for his efforts.
For many South Africans, the elder statesman is also thought of as a beloved family member and referred to by his clan name, Madiba.
Students at the Johannesburg School for the Blind and Partially Sighted sang and danced on Monday morning as they wished their "Tata Mandela" a happy birthday.
President Jacob Zuma, who planned to visit Mandela in Qunu after meeting with visiting British Prime Minister David Cameron in Pretoria, used the occasion to call for greater efforts to end poverty.
"We have achieved a lot, but we must still work further to eradicate poverty and improve especially the lives of children, because Madiba loves them so much," Zuma said, using Mandela's clan name.
Zuma was also expected to launch a drive to increase membership in the ruling African National Congress "as part of realising ANC stalwart Mandela's dream and that of his organisation."
UN Secretary General Ban Ki-moon urged the world to observe Mandela's call to carry out volunteer work.
"Everybody remembers and, indeed, needs an inspirational figure who has played a signal role in their lives. Nelson Mandela has been that role model for countless people around the world," Ban said.
Increasingly frail with age, Mandela was last seen in public just before his 92nd birthday, when he and his third wife Graca Machel made an appearance at the football World Cup final in Johannesburg.
As South Africa's first black president, Mandela is revered for having ushered in democracy and for his personal sacrifices in fighting the apartheid regime.
On his release in 1990, he led negotiations that paved the way to elections in 1994. He used his warmth, dignity and self-deprecating humour to help heal racial divisions and opened a process of reconciliation.