HARARE (Reuters) - Zimbabwe’s first lady, Grace Mugabe, on Friday made her first public appearance since leaving South Africa where she was charged with assault.

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The wife of 93-year-old Robert Mugabe, a potential successor to the president, was granted diplomatic immunity and left South Africa on Sunday.

But South Africa’s opposition is challenging the international relations minister’s decision to give her immunity - a move that could in theory affect any future plans to travel to the country.

The Mugabes attended a farming fair in Harare, where Grace was seen smiling and talking with exhibitors. She made no public comment.

Harare has made no official comment on the assault case and requests for comment from Zimbabwean government officials have gone unanswered.

Twenty-year-old model Gabriella Engels accused Grace Mugabe of whipping her with an electric extension cable as she waited with two friends in a luxury hotel suite to meet one of the Mugabes’ adult sons.

South Africa’s main opposition Democratic Alliance (DA) party has asked the Constitutional Court for permission to argue its case that Grace Mugabe’s diplomatic immunity be declared unconstitutional.

“Grace Mugabe is not a member of the Zimbabwean government and she was in South Africa on personal business,” the party’s federal executive chairman, James Selfe, said.

“There is nothing in either South African or international law which renders her deserving of diplomatic immunity.”

Selfe said the party hoped that if the immunity is lifted Mugabe would have to surrender herself to South African state prosecutors or be extradited to face the charges. If not she could be declared persona non grata, prohibited from visiting South Africa.

Advocacy group Afriforum has given legal backing to the alleged victim, Engels, and is working on the case with Gerrie Nel - the prosecutor who secured a murder conviction against Olympic and Paralympic runner Oscar Pistorius.