The rape allegations against Cosatu general secretary Zwelinzima Vavi are part of a "dirty tactics" campaign by his political opponents within Cosatu to destroy him, unions said on Monday.

"The timing of this claim is worrying and it reinforces our long-held view that a sinister campaign is being used to push Vavi out from his elected position for narrow political and selfish ends," the National Union of Metalworkers of SA (Numsa) said in a statement.

"We fully support Vavi during this difficult and testing period since he has not been found guilty by any court."

Vavi has admitted to having sex with a married woman, who was employed by the Congress of SA Trade Unions (Cosatu), and alleged she later tried to extort R2 million from him.

He denied raping her, and said the sex was consensual.

However, she brought a complaint of sexual harassment against him. An internal grievance proceeding was being held on Monday.

Numsa said Vavi's public apology about the matter was a sign of his bravery and courageous leadership style.

However, the union viewed rape as a serious offence and encouraged Vavi's accuser to open a case with the police.

The SA Municipal Workers' Union (Samwu) said Vavi was being targeted because he had "dared to speak the truth to power".

"He [Vavi] has dared to speak the truth on unemployment, the absence of service delivery, corruption, and many other issues... and in so doing, has incurred the wrath of those with career ambitions," the Samwu said in a statement.

The union said the internal processes at Cosatu should be allowed to run their course.

Earlier on Monday, the National Union of Mineworkers (NUM) said it was not in a position to comment on the matter and that it was waiting to hear from Cosatu.

Sapa