A new Chinese app that lets users swap their faces with celebrities, sports stars or anyone else in a video clip racked up millions of downloads at the weekend but was swiftly criticised over privacy issues. The app's surge in popularity and sudden backlash from some users highlights how artificial intelligence (AI) technologies raise concerns surrounding identity verification. Zao was uploaded to China's iOS App Store on Friday and immediately went viral. According to a post from the app's makers on China's Weibo social media site, Zao's servers nearly crashed because of the deluge of traffic. According to App Annie, a firm that tracks app downloads all over the world, Zao was the most downloaded free app in China's iOS App Store as of 1 September.