The app’s surge in popularity and sudden backlash from some users highlights how artificial intelligence (AI) technologies bring about new 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 makers on China’s Twitter-like Weibo, ZAO’s servers nearly crashed due to the surge in 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 Sept. 1.

Consumers sign-up for ZAO with their phone number and upload images of their face, using photographs taken with their smartphone.

They can then choose from a range of videos of celebrities on which to superimpose their face, and share the videos with their friends.

In addition to Chinese celebrities, other famous faces on the app include Leonardo DiCaprio and Marilyn Monroe.

Gu Shi, a 21-year-old student in Shanghai, downloaded ZAO after seeing her friends post clips on their WeChat feeds.

“I’ve never tried using Japanese makeup and hairstyles because it’s too complicated to do all by myself,” she told Reuters.

“This app gave me a chance to try a totally different style from my normal life.”

As the app went viral, some users complained that its privacy policy could endanger them.

One section of the user agreement stated that consumers who upload their images to ZAO agree to surrender the intellectual property rights to their face, and permit ZAO to use their images for marketing purposes.

Zao said on Weibo that it would address those concerns.

“We thoroughly understand the anxiety people have toward privacy concerns,” the company said. “We have received the questions you have sent us. We will correct the areas we have not considered and require some time.”

ZAO was published by Momo Inc, best-known as makers of a dating app that later transformed into a livestreaming service. The company listed on the New York Stock Exchange in 2014.

Momo did not immediately respond to Reuters’ requests for comment.