Cosatu congress delegates chanted "Zuma, Zuma" as President Jacob Zuma made his way on to the stage at Gallagher Estate in Midrand on Monday.
Wearing a red golf shirt, he hugged a few people before taking his seat at the Congress of SA Trade Unions' (Cosatu) 11th national congress.
Zuma was expected to address delegates after Cosatu president S'dumo Dlamini had delivered the opening address.
SA Communist Party general secretary Blade Nzimande was expected to speak later in the day.
Delegates from Cosatu's 20 affiliates were scheduled to discuss the organisation's political report on Monday.
They were also expected to begin the process of electing national office bearers. The election results will be announced on Thursday, the final day of the congress.
Congress house rules do not allow singing divisive, war, or tribal songs, "unrelated to the class battle".
"Singing in praise of any serving leader of the alliance is not divisive.
However, singing against any leader of the alliance... is divisive and will not be allowed," according to the rules.
The rules also ban "showing of signs that seek to prematurely open the debate on leadership in the ANC".
Before the congress opened, a group of delegates outside sang while showing two fingers in the air - a sign calling for Zuma to serve a second term as African National Congress president.
The ANC holds its national conference in Mangaung in December, when new leaders will be elected. Nominations for leadership positions by branches officially open only in October.
The Cosatu congress is reportedly expected to debate the leadership battle.