Riot police appeared in the city after the fee was introduced.Getty Images News/Getty ImagesLast week, Venice introduced a €5, or $5.40, fee for day-trippers to try and mitigate overtourism.The fee is part of a trial that will run until July and apply on 29 "peak" days.The move sparked protests, highlighting the problems city officials face when tackling overtourism.Venice is among many cities grappling with the effects of overtourism as the number of visitors bounced back from pandemic lows.