A South African civil rights organisation said Monday it would challenge the government's decision to grant diplomatic immunity to Zimbabwe first lady Grace Mugabe, who was facing assault charges.

The wife of President Robert Mugabe allegedly attacked a 20-year-old model with an electrical extension cord at a hotel in Johannesburg where the couple's two sons were staying.

The attack left Gabriella Engels with cuts on her forehead and on her head that required stitching and she filed an assault charge against Mugabe.

South Africa's foreign affairs ministry said on Sunday that it had granted Mugabe immunity after "careful consideration of all the relevant factors".

AfriForum, which helps victims of crime, is preparing an court injunction to set aside the foreign ministry decision.

"We are going to ask the court to say that the decision to grant immunity is unlawful because our legislation says that diplomatic immunity cannot apply in serious crimes, and violence is part of that," AfriForum's chief Kallie Kriel told AFP.

Mugabe, who had been sought by police, flew out of South Africa in the early hours of Sunday, hours before the foreign ministry announced that it had granted her immunity.

The decision to grant Mugabe immunity "was not an easy one to make," it said.

AfriForum has vowed to help Engels seek justice and denounced what it said was a regional "culture of impunity".

Seeking diplomatic immunity was an "afterthought," said Kriel.

"She had been here for medical assistance with regards to her foot and therefore diplomatic immunity does not apply."

"We believe that the diplomatic immunity should not have been granted".

The group plans to launch a private criminal prosecution, which is allowed under South African law. They intend for it to be led by Gerrie Nel, the former state prosecutor who led the case against Paralympian Oscar Pistorius and has a reputation of winning high-profile cases.

Even if the effort fails, Kriel hopes it may stop Grace Mugabe visiting South Africa.

"She loves shopping in Sandton in Johannesburg, so in the worst case scenario if we get to a situation where she cannot come to the country any more, I think that will be a positive step."

"But the first prize is to make sure she is being prosecuted," said Kriel.

source: AFP