A global journalism watchdog targeted Economic Freedom Fighters (EFF) leader Julius Malema on Saturday after he tweeted the personal phone number of a reporter who has since received a flood of threats.
"The Committee to Protect Journalists today expressed grave concern about threats and harassment against veteran South African journalist Karima Brown," said the organisation which runs a global network from New York.
The statement came after Malema tweeted Brown's telephone number, also calling her a "mole" for South Africa's ruling ANC party.
Brown said that within hours she received a flood of messages she believes were sent by activists of the EFF, South Africa's third-largest party.
Some threatened to rape her, or burn her alive.
"We are concerned about Karima Brown's safety given the threats instigated by the malicious publication of her contact information," said Angela Quintal, CPJ's Africa programme coordinator.
South Africans vote in a general election in May, with the ruling ANC party widely expected to conserve its absolute majority.
"The electoral commission must send the Economic Freedom Fighters and all political parties a strong message that using social media to threaten and intimidate journalists will not be tolerated and is a breach of the Electoral Code that could impact the credibility of the vote," she said.
Malema himself has since denounced the most extreme tweets sent to Brown, who works for the eNCA network and the independent radio station Johannesburg 702, but he and his party continue to accuse her of being partial.
His initial message has been deleted on Twitter.
Malema, a former ANC member himself, is known for controversial statements, prompting several legal complaints.