Qi Wei, an actress, holds an A4 paper. The message reads: "Verification codes" should not be given to others, even to your parents! [Photo/Weibo]

Police have now thrown their weight behind the latest trend of showing off slim waists. But don't panic, it's all for a good cause.

After the "A4 waist challenge" – where online users post with A4-size sheets of paper placed in front of their midriffs to show off their slim bodies – went viral, police decided to fill the blank sheets with some social messages.

The "challenge", which began in China in February, has seen many, including actresses Yuan Shanshan and Qi Wei, posting photos on Weibo to show off their waistlines.

If the waist is completely hidden behind the 21-centimeter paper, it is proof that the person has a thin waist, and a thin waist is often considered a symbol of beauty.

In an effort to raise awareness and take advantage of the viral photos, police used a software to highlight anti-fraud messages. Here's a look at some of the images.

Guo Jingfei, an actor, holds an A4 paper. The message says: An incoming call that doesn't display the number? – Do not answer it. [Photo/Weibo]