But it claims that South African advertising companies have refused to erect the billboards because of the influence of the telecoms giant.
United Against Nuclear Iran, an organisation founded by the late United States ambassador Richard Holbrooke, claimed this month that MTN, which has a 49% stake in Iran's second-biggest cellphone network, has provided the regime with the means to suppress and track dissenters, some of whom are later tortured.
A lawsuit filed in the US by Turkcell, MTN's rival, echoes these claims and alleges further that the company has supported Iran's illegal nuclear programme.
MTN not only denies all claims of corruption and enabling oppression, but also claims that its cell service to 38-million subscribers in Iran – "many under 21 [years old]" – actually "widens political freedoms".
This week, Nathan Carleton, spokesperson for the lobby group, told the Mail & Guardian that, since early May, artwork and an initial undisclosed budget was offered for the erection of billboards opposite MTN's headquarters in Gauteng. The advert shows a picture of Iranian plain-clothes officers beating a civilian with clubs, alongside the words: "MTN helps the Iranian regime terrorize and oppress its citizens."
But, at least half a dozen South African advertising companies have refused to print and erect the advert, including Adreach, which called the campaign "distasteful", Media24, which cited a conflict of interest in its business with MTN, and Outdoor Media, which said it even feared for its staff's safety, owing to "potential retaliation from other parties". Only Alliance Media is still considering running the campaign.
Carleton said its US campaign had already turned some potential US business partners against MTN. But, he said: "We wanted to bring the campaign 'home' to South Africa … where the vast majority of MTN's employees and customers would be appalled to know the full extent of MTN's alliance with the world's largest state sponsor of terrorism.
"We made it clear to our representatives that we were ready to pay the regular or even the premium rate for this billboard, but we were rebuffed each time. We tried for months, but clearly MTN's presence in South Africa and its advertising prowess have caused a chilling effect on what can be published about it."
In 2010, United Against Nuclear Iran succeeded in forcing equipment conglomerate Caterpillar to withdraw its subsidiary involvement in Iran following a similar and highly successful billboard "shame campaign" in the US.
The lobby group claimed a subsidiary's equipment was being used to bore tunnels for Iran's nuclear facilities. One giant billboard that faced its headquarters in Illinois in the US stated, next to a picture of a Caterpillar crane: "Today's work. Tomorrow's nuclear Iran?"
A US advertising agent, who asked not to be named, said hundreds of thousands of rands had been made available for the launch of the anti-MTN campaign in South Africa.
The lobby group recently stepped up its war against MTN with a string of new condemnations, including outraged editorials on the fact that MTN sought assistance from the US government to extract its profits from Iran.
Last Friday, Mark Wallace, a former US ambassador to the United Nations and now the lobby group's chief executive, called MTN "a direct threat to US national security interests" and called on the White House to impose sanctions on the company.
"It is unacceptable that MTN, a complicit partner of the Iranian regime, is still able to raise capital in the US and has not had its assets and property blocked by the US government," said Wallace.
Paul Norman, a spokesperson for MTN, said: "The adverts are meant to shock, are offensive to MTN employees and malicious. MTN denounces this advert and United Against Nuclear Iran's efforts to place it on billboards in South Africa. MTN is considering how best to address the issue, including legal action."
Cherne Botes, country specialist for Alliance Media, said a 4.5m-by-18m billboard on the Soweto highway had been set aside for the advert, but another company had taken the space after delays in getting the lobby group's graphic.
Simon Haskell, a director with Outdoor Advertising, admitted that his company had declined the campaign partly because it feared for the safety of its staff and equipment.
He said: "MTN is a client of ours and has been for more than five years. We felt it unethical to turn on it in this manner." Haskell said he was also worried about accuracy and fairness: "Although we understood the message of the campaign, we were not sure of the facts."
Mark Castel, national sales manager for Adreach, said he refused the advert immediately because it was offensive: "We think the content of the advert will be distasteful to the South African public and … it probably amounts to hate speech."
Castel denied any pressure from MTN to reach this decision, saying: "MTN is not a large customer of ours. It accounts for less than 0.3% of our turnover, so the suggestion that we are protecting income is wrong."
Carleton said the lobby group had lodged an official complaint with the Advertising Standards Authority, alleging that its campaign was being unfairly sabotaged.
Said Norman: "MTN has no involvement with human rights abuse in Iran. It does not condone or support the use of technology by any government to oppress its citizens. It has no knowledge of, or involvement in, any alleged or illegal activity by the Iranian government nor security authorities."
Fred Makgato, head of legal affairs for the Advertising Standards Authority, said the agency could not comment on the advert until it was flighted and complaints were received. "Corporate or media houses have the choice whether to flight an advert or not. Recently, we have seen the SABC and Multichoice refusing to accept advertising from Nando's due to its sensitivity on xenophobia," Makgato said.