City is Rome - River is Tiber and competition is Where in the World? Daily Telegraph.
I would have thought the reference to Carabinieri would have given it away.
"in full ceremonial dress, members of the Carabinieri stroll through a city that was formerly at the centre of a vast empire that spanned three continents. "
Must be Italy and a reference to the Roman Empire, because Carabinieri are the gendarmerie of Italy and not Turkey :-)