JOHANNESBURG, South Africa — The archbishop of Bulawayo — the most outspoken and enduring critic of the Zimbabwean president — stepped down yesterday over a sex scandal. The resignation was seen as a stunning propaganda coup for Robert Mugabe.

Archbishop Pius Ncube, who has in the past been quoted as saying that a British invasion would be justified to oust Mr. Mugabe, stepped down two months after allegations emerged of an affair with a church employee, Rosemary Sibanda.

Mrs. Sibanda’s estranged husband sued him for about $160,000 in damages for “loss of companionship,” and the scandal was given blanket coverage in the state press, with full pages of broadsheet newspapers devoted to grainy photos allegedly showing the cleric naked in bed with the woman.

More explicit images supposedly of the couple, said to have been recorded in the archbishop’s bedroom above the Roman Catholic cathedral in Bulawayo, were shown on state television over several nights.

It is widely believed that Zimbabwe’s security service was involved. The photographs were taken by a local private investigator said to be a former member of the Central Intelligence Organization during state-sponsored massacres of opposition supporters in the 1980s. But the archbishop’s claim to the moral high ground was left compromised.

Mr. Mugabe — who like the cleric was educated in a Jesuit school — took to publicly joking about the archbishop, adding that he would pray for him.

The archbishop was on a week-long retreat of “prayer and reflection” yesterday and could not be contacted.