A viral Chinese app which lets users swap their faces with celebrities in video clips has come under fire over claims its privacy policy had put millions of people's data at risk.

Zao, which became available to download in China on Friday, racked up millions of downloads over the weekend and, according to tracker App Annie, became the most downloaded free app in the China iOS App Store as of September.

It is not yet available to UK or US customers, or anyone without a Chinese phone number.

The app lets users upload pictures of themselves into the system, and then swap their faces onto celebrities' bodies, being able to choose from popular scenes in hundreds of TV shows or films. Many of the celebrities on the app are Chinese stars, although choices also include Leonardo DiCaprio and Marilyn Monroe.

However, the app faced a backlash over the weekend, after users complained that its privacy policy was inadequate.