Benias Gwenhamo Madhakasi, who works as a street vendor in South Africa, was arrested at the Beit Bridge border post on April 29 on charges of insulting or undermining the authority of the president.

He was denied bail and held in custody until Tuesday, when a magistrate tossed out the case, said Zimbabwe Lawyers for Human Rights, which represented Madhakasi.

"Prosecutors claimed that Madhakasi was found in possession of skeletal nude pictures portraying President Mugabe in his mobile phone handset," the lawyers said in a statement.

One of the pictures had a caption which read, "Happy 87th birthday Robert Mugabe," the prosecutors said.

Mugabe, who turned 88 in February, is Africa's oldest ruler.

Beit Bridge magistrate Auxillia Chiumburu dismissed the case as "a fishing expedition", after police changed the charges to immigration violations.

There are regular reports of arrests for slandering Zimbabwe's long-time president and breaching the strict Public Order and Security Act.

Usually those found guilty receive light jail sentences, fines or are ordered to do community service.