Anti-apartheid hero Nelson Mandela celebrated his 94th birthday in "good" spirits on Wednesday as millions of South Africans joined in the global praise for the much-loved statesman.

Mandela, whose 1990 release from an apartheid prison put South Africa on the path to democracy and reconciliation, spent a quiet day with his close family in his home village of Qunu, where he has retired from public life.

"He is in good spirits and looking very well," granddaughter Ndileka Mandela told AFP by phone from Qunu.

From the variety of his favourite dishes on offer for a birthday lunch that included tripe, corn samp and dumplings, Mandela chose fish and vegetables, said Ndileka.

His wife Grace Machel and ex-wife Winnie Madikizela-Mandela, as well as his daughters were among those that had lunch with him.

He enjoyed the love and attention.

"He likes to have the buzz around him," said Ndileka.

South Africa's 12 million schoolchildren kicked off the day singing happy birthday to the anti-apartheid hero and Nobel Peace Prize laureate.

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.

"As we celebrate his birthday, we remember that he fought for us in the struggle. The struggle was to fight apartheid," said 12-year-old Kgaugelo Masunhloane at Batsogile primary school in Soweto.

"He spent 27 years in prison. We would want to say thank you Madiba, and we wish you happy birthday, and many more years to come."

Mandela, known fondly by his clan name Madiba, has not appeared in public since the World Cup in 2010.

"Generally, he's looking good and he's joking, he's his old self," another of his granddaughters Tukwini Mandela told CNN from the family compound.

Images of Mandela have become rare since he retired to Qunu.

He, however, retains the ability to inspire, and for many South Africans remains a symbol of the best of themselves, for leading the nation away from civil war and to a non-racial democracy.

But fellow Nobel peace laureate Archbishop Desmond Tutu warned that South Africa was not living up to Mandela's ideals, saying he would be shocked at the shambolic state of public schools.

"If he (Mandela) knew what was happening he would be crying... it's totally unacceptable," Tutu told a leadership summit in the central Free State province.

He was angry at the failure by education officials to deliver textbooks to some rural schools, noting that some children were still learning under trees because of a lack of classrooms, 18 years after Mandela became South Africa's first black president.

At the Tour de France, children unfurled a banner to wish the icon happy birthday at the start of the daily stage.

And the science world added an unusual tribute, naming a prehistoric woodpecker -- the oldest type ever found in Africa -- after Mandela.

President Jacob Zuma pledged to uphold Mandela's legacy of promoting forgiveness, unity and a non-racial society.

"We make a commitment to continue where Madiba left off. We will continue the long walk to confront poverty, unemployment and inequality," he said in a statement.

At 94, Mandela is just six years younger than the ruling ANC party, which has been rocked by bitter in-fighting for the last five years.

Madikizela-Mandela who last week castigated the ANC over "shabby treatment" of the Mandela family earlier said: "We cannot pretend all is well in the African National Congress," as she marked the birthday at an orphanage.

"That is not the ANC he gave up his life for and that is not the ANC we all wanted it to be. Especially on his birthday."

Mandela was born on July 18, 1918 in a Thembu royal family in Mvezo village southeastern South Africa.