Nearly 12 million school children across South Africa have kicked off celebrations marking the 94th birthday of Nelson Mandela, the country's deeply loved anti-apartheid icon, with resounding choruses of Happy Birthday.

School assemblies began Wednesday with birthday wishes for Mandela who is expected to spend the day privately at his homestead in the southeastern province with his family.

Tributes to Mandela were pouring in: U.S. President Barack Obama said Mandela "has changed the arc of history, transforming his country, the continent and the world."

Communities in South Africa and around the world will dedicate 67 minutes of the day to charity work and projects for the needy to mark Mandela's 67 years in activism.

South African churchmen and politicians urged people across the country `'to make every day a Mandela Day."