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HARARE (Reuters) - Zimbabwe’s first lady Grace Mugabe hit the campaign trail with her husband Robert on Friday, urging discipline in his party two weeks after she faced assault charges in South Africa.

Grace, who is seen as a possible successor to her 93-year-old husband, was accused of assaulting a model at an upmarket Johannesburg hotel, but flew home after a minister granted her immunity.

The first lady did not refer to the incident in her first speech since returning, instead telling a ZANU-PF party rally that supporters should stand behind Africa’s oldest leader in the build-up to next year’s elections.

“We have a very unique position in Zimbabwe where we have our president who will soon be 94 years because that is what God decreed. No man of flesh can stop that,” Grace said in the speech broadcast on state television.

South Africa’s main opposition Democratic Alliance (DA) party 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.

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.