LUXEMBOURG — The British, Luxembourgish and European Union flags, ironed and pristine, were set behind two lecterns on a warm, sunny afternoon in the cobbled courtyard outside the Luxembourg prime minister’s office.

The press was arranged in a semicircle, cameras at the front, ready for a news conference about negotiations over Britain’s expected departure from the European Union.

But when Prime Minister Boris Johnson of Britain and Luxembourg’s Xavier Bettel, his counterpart, appeared, they could hear the loud boos from a few dozen protesters behind a fence, who were chanting, “Tell the truth, stop the coup!”

Mr. Bettel walked Mr. Johnson to the exit and returned alone, visibly agitated.

Then he took questions, gesticulating toward an empty lectern and chiding an invisible Mr. Johnson.