The ANC is almost certain to win the election but faces its biggest challenge since coming to power when apartheid ended in 1994.
Former president Mandela, 90, was driven into a packed sports stadium in a golf cart to cheers from the tens of thousands of ANC supporters gathered for the rally to be addressed by party leader Jacob Zuma.
A frail-looking Mandela, wearing an ANC T-shirt, was helped onto the stage by Zuma.
His attendance at the rally is bound to give the ANC a boost days ahead of the election in which analysts expect the ruling party to lose its two-thirds parliamentary majority as it faces criticism over its performance on poverty, crime and AIDS.
The new breakaway Congress of the People (COPE) party, formed by ANC dissidents, and a revitalised official opposition Democratic Alliance hopes to tap into frustrations with ANC graft scandals.
The ANC said its final campaign rally will be one of the biggest political rallies to be held in the country yet. It is being held at two adjoining Johannesburg sport stadiums with a combined capacity of over 100,000 people.
State prosecutors have given the ANC a boost by dropping graft charges against Zuma, whom the new parliament is certain to elect president. Zuma has said the graft case was part of a political campaign to undermine him.
His ANC has promised to do more to bring economically disadvantaged blacks into the mainstream economy through land reform and affirmative action programmes.
But Africa’s biggest economy is on the brink of recession, and Zuma will be in a difficult position. Union allies are pushing him to spend more on the poor, while foreign investors crucial for growth fear he will steer the economy to the left.