BEIJING — Women in China are sending naked photographs of themselves holding their ID cards to peer-to-peer lenders as collateral, Chinese state media have reported, in a trend that may involve thousands of people.

The phenomenon, which the Beijing Youth Daily said was taking place on a popular online peer-to-peer lender called Jiedaibao, is provoking alarm and sympathy on social media. It has also led to calls for women to protect themselves from predators who are reportedly blackmailing them with the nude photographs, sometimes demanding sexual services when the women cannot repay at interest rates that can be as high as 30 percent a week.

A call seeking comment to Jiedaibao, a leading company in China’s flourishing peer-to-peer lending market, was not answered late on Wednesday in Beijing.

Beijing Youth Daily, the official newspaper of the Communist Youth League, said the practice was an “open secret.”