A 21-year-old policeman was detained yesterday after firing his service revolver in the air following a dispute with three young men in a car in the central Athens district of Ambelokipi. The 21-year-old officer said he had been driving a patrol car down Kifissias Avenue when a vehicle, in which the three youths had been traveling, overtook him aggressively, near the President Hotel, breaking off one of his wing mirrors. The officer said he then pulled up alongside the youths' car seeking an explanation but that they started threatening him. He said he fired his gun in the air to scare them off. The youths in the car fled. In a related development yesterday, a 30-year-old security guard shot by a policeman last week outside the home of the US ambassador was transferred from the intensive-care unit of the Athens hospital where he is being treated to a regular ward.