Former president Jacob Zuma's resignation did not mean he should be able to evade prosecution, AfriForum said on Wednesday.
CEO Kallie Kriel says the organisation would "take all reasonable steps" to ensure that Zuma is prosecuted by the National Prosecuting Authority (NPA).
"If the NPA fails to prosecute Zuma, AfriForum's private prosecution unit, under the direction of advocate Gerrie Nel, puts in motion the process of private prosecution," he warned.
AfriForum welcomed Zuma's late-night announcement that he would resign with immediate effect.
The ANC caucus met on Wednesday morning on the outcomes of a meeting of the party's national executive committee, and ANC treasurer general Paul Mashatile told the media that they had given Zuma until Wednesday to resign.
If Zuma did not resign, he said the ruling party would have proceeded with a motion of no confidence against him on Thursday to legally force him to resign according to the Constitution.
ANC secretary general Ace Magashule and his deputy Jessie Duarte delivered Zuma's recall letter to the presidential guesthouse on Tuesday morning.