ANC party officials had expressed hope the 93-year-old anti-apartheid icon would attend the festivities.
Party secretary-general Gwede Mantashe announced Friday that Mandela will miss the activities, explaining "he is in good spirits, but very, very old."
Mandela and the ANC led the struggle against the apartheid system of white-minority rule. In 1994, he became South Africa's first black president, and the ANC has ruled the country ever since.
Celebrations began on Friday and the main event is scheduled for Sunday, the ANC's official 100th anniversary.
ANC spokesman Jackson Mthembu told VOA, English to Africa, more than 40 heads of state from Africa will join the Sunday's festivities in Mangaung, Free State Province. The ANC was founded in the village on January 8, 1912.
The ANC says events this weekend will celebrate the party's core principle of allowing people to chose their own destiny in a "united, non-racial, non-sexist nation."
While the ANC has ruled in South Africa since Mandela's election, its popularity has eroded in recent years. Critics say the party has failed to tackle corruption and deliver basic services to South Africans.
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