Millions of South African schoolchildren sang happy birthday to Nelson Mandela as he turned 93 Monday, while politicians and ordinary citizens did charity work to support his call to do good.

For the third year, at the request of his charitable foundation, July 18 is observed as Mandela Day, recognised by the United Nations as a global call to volunteer for good causes for 67 minutes -- representing each year of Mandela's life in active politics.

The nation's 12.5 million schoolchildren sang "Happy Birthday" before starting class Monday, with television and radio stations urging the nation to join in the special rendition of the song, given an African twist by a local composer.

"He must relax and must not get sick. He must live long," said 10-year-old Kabelo Masemola, whose Johannesburg school suspended extra-curriculars for the week so the students can do volunteer work instead.

Tributes also came from world leaders, with British Prime Minister David Cameron saluting Mandela's legacy during his visit to South Africa on Monday.

"President Mandela is an inspiration to the world, and as we celebrate his birthday and look back at just how far South Africa has come, so I believe we can look forward with confidence to an even better future for South Africa and her people," Cameron said.

US President Barack Obama hailed Mandela as "a beacon for the global community, and for all who work for democracy, justice and reconciliation."

"Mandela's legacy exemplifies wisdom, strength and grace, and on the anniversary of his birth we salute the example of his life," he said.

Archbishop Desmond Tutu, who like Mandela has won the Nobel Peace Prize, said he had sent good wishes to Mandela and his wife Graca Machel, as the couple is also celebrating their 13th wedding anniversary.

"On behalf of The Elders, I sent him an email because it's also their wedding anniversary," Tutu told reporters.

Tutu and Machel both belong to The Elders, a group of global statesmen.

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.

Mandela spent his birthday with his family at his childhood village Qunu in the Eastern Cape province. Pictures released by his foundation showed him gathered with his family around an enormous birthday cake.

Later in the day, President Jacob Zuma visited Mandela in Qunu and showed him a model of a bridge to be built nearby.

His grandson, chief Mandla Mandela said the family would give blankets to elderly people around his birthplace of Mvezo.

"We'll be ensuring that at least our elderly citizens are looked after and taken better care of, but we are also having kids from the school in Mvezo that will be cleaning the community because we want to ensure that we preserve our area," he told national broadcaster SABC.

Similar activities were underway around the country, as South Africans cleaned and painted schools, orphanages and clinics, while others donated food, clothes, books and toys to charity.

Increasingly frail with age, Mandela was last seen in public just before his 92nd birthday, when he and 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.