Acclaimed Zimbabwean author Tsitsi Dangarembga has been convicted of inciting violence by carrying a placard calling for political reform.
The magistrate said the protest could have incited other people to join, and cause a breach of peace.
Dangarembga paid a fine of about $110 (£100) to avoid serving a three-month jail term.
She had pleaded not guilty in a trial that critics say is the latest sign of a government crackdown on dissent.
Outside the court, Dangarembga told the BBC she was not surprised by the conviction.
“The space for freedom of expression and freedom of the media is shrinking and increasingly criminalised” in Zimbabwe, she said, adding that she intended to appeal against the judgement.
When President Emmerson Mnangagwa took power in 2017, he vowed to introduce reforms after the decades-long repressive rule of his predecessor, Robert Mugabe, however critics say little has changed.