MEXICO CITY — A few weeks ago, prosecutors in Venezuela won the conviction of a prominent opposition leader who had been charged with inciting violence during antigovernment protests. It was widely seen as a sign that the government of President Nicolás Maduro would continue to pursue a hard line against its opponents — and critics inside and outside Venezuela condemned the trial as being politically motivated.

Now, in an unexpected turn that has the country buzzing, Franklin Nieves, one of the two main prosecutors in the case against the opposition leader, Leopoldo López, has released a video in which he calls the trial a farce and says that he has fled the country.

“I decided to leave Venezuela with my family because of the pressure that I was under from the executive branch and my superiors to continue to defend the false evidence that was used to convict Leopoldo López,” Mr. Nieves says in the video, which was posted on a popular Venezuelan website.

Mr. Nieves says the pressure came as he was expected to help defeat an appeal filed by Mr. López’s lawyers. Mr. López was sentenced to more than 13 years in prison.