HARARE (Reuters) - Zimbabwe's First lady Grace Mugabe returned from South Africa early on Sunday (Aug 20), state media reported, after a 20-year-old model accused her of assault in an upmarket Johannesburg hotel room.

South African police had placed border posts on "red alert"to prevent her leaving the country, but a security source told Reuters on Friday that the wife of 93-year-old President Robert Mugabe had been granted diplomatic immunity.

Zimbabwean state media reported that the president and first lady arrived in Harare aboard an Air Zimbabwe plane in the early hours of Sunday. A source confirmed this to Reuters.

A bruised Gabriella Engels has accused Grace Mugabe of whipping her with an electric extension cable a week ago as she waited with two friends in a luxury hotel suite to meet one of Mugabe's adult sons.

President Mugabe attended a South African Development Community summit in Pretoria on Saturday, but his 52-year-old wife was not there or part of his delegation.

Harare has made no official comment on the saga and requests for comment from Zimbabwean government officials have gone unanswered. The South African government has restricted all official comment to the police ministry.