Indian-origin Pravin Gordhan was speaking at a memorial service held in Durban in honour of anti-apartheid icon Ahmed Kathrada.
Youth members of the ruling African National Congress booed Gordhan and disrupted his speech with song and dance.
President Jacob Zuma fired Gordhan nine days ago, fuelling divisions within the ANC party which has ruled South Africa since the end of apartheid rule two decades ago.
"If we can't unite among ourselves there is no hope for us in 2019," Pravin told hundreds of people, most of them members of the ANC youth league wearing the party's yellow T-shirts and some carrying pro-Zuma placards.
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The dismissal of Gordhan saw the Fitch and Standard & Poor's agencies cut South Africa's sovereign credit rating to junk status last week.
The reshuffle also heightened calls for Zuma to resign, with several senior ANC figures criticising Gordhan's removal.
ANC treasurer-general, Zweli Mkhize, one of the party's top six leaders - also spoke at the memorial service held in Zuma's stronghold of the eastern KwaZulu Natal province, but his speech was drowned by pro-Zuma chants and songs.
His funeral and other subsequent memorial services have turned into an anti-Zuma rallies with speakers drawing attention to the growing discontent within the ANC.
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