The group's soft approach to Zimbabwe's president disappoints many, but some say it may be more effective in engaging him.
The labor-friendly Zuma is facing pressure on wages and job security. Public anger has led to riots in poor neighborhoods.
So many voters flocked to the polls that many stations ran out of ballot papers.
The polarizing ANC leader is expected to draw record numbers of voters to the polls in Wednesday's presidential race.
South Africa's ruling party candidate, Jacob Zuma, vows no constitutional changes ahead of April 22 vote.
The International Criminal Court issued an arrest warrant Wednesday for President Omar al-Bashir on charges of war crimes and crimes against humanity.
Wednesday's swearing-in ceremony, officiated by President Mugabe, put an end to 10 months of political stalemate.
Opposition leader Morgan Tsvangirai agreed last Friday to form a power-sharing government with longtime President Robert Mugabe.
South Africa's president was ousted by his own party this weekend.