"We see this as a mischievous warning … directed at the ANC," he told reporters at Luthuli House on Tuesday.
"Sectors of society want a particular outcome … I don't think we should accept that."
Mantashe said he was disappointed by the banking, business and cleric sector as they were trying to change the outcome of the conference.
He was responding to reports that religious leaders had written to President Jacob Zuma, warning that the public was concerned that South Africa had lost its moral compass.
In the letter, clerics expressed the view that "our democracy can be significantly improved".
The letter was signed by Archbishop Thabo Makgoba of Church Leaders Consultation, Bishop Joe Seoka of the South African Council of Churches, Reverend Moss Ntla of the Evangelical Alliance of South Africa, and Reverend Edwin Arrison of Kairos Southern Africa.
"They [South Africans] yearn for a change from an increasingly corrupt political, business and societal culture to one that is accountable to the people," the clerics wrote.
Copies were sent to political and economic leaders, including Democratic Alliance leader Helen Zille.
The presidency confirmed that Zuma had seen the letter. The ANC is scheduled to hold its elective conference in Mangaung next week.
The clerics said the public was not interested in "a superficial change of one self-serving political leader for another, or one political party for another".
Mantashe said the ruling party would not allow any sector to write the ANC's policy. "It is a question of principle."
He said the determination of the party's leadership should be respected by all sectors.
"The ANC must be allowed to run its business." – Sapa