Moldova’s police chief, appointed by a government committed to joining the European Union and leaving Russia’s orbit, was alarmed to find his country’s capital suddenly plastered with posters bearing a blunt message: “No EU.”

The posters — written in Russian and Romanian, Moldova’s main language — appeared overnight on bus stops across Chisinau last month, ostensibly part of an advertising campaign for a concert by a popular Russian-speaking singer from Ukraine.

The timing, however, set off alarm bells: the anti-E.U. message came just as Moldova, a former Soviet republic, was gearing up for a contentious referendum on whether to amend its constitution to enshrine the “irreversibility” of its “European course.”

Now, with just days left before voting on Sunday, the police chief, Viorel Cernauteanu, says he knows what was going on.

The posters had nothing to do with the singer, he said in an interview, but were part of a “big psychological operation” directed by Russia to derail the referendum, which will be held at the same time as a presidential election.