The pitches frequently appeal to aspiring small investors like Walter Xu, a recent university graduate who was drawn to Qianbao by promises of sky-high returns. Qianbao looked like a real business: Its portal sold cellphones and appliances — with discounts for members — as well as big returns for those who gave it money. In exchange for depositing money, watching ads and writing reviews, it offered returns of as much as 50 percent. In all, Mr. Xu invested $32,000 of his savings in Qianbao.

On Dec. 26, when Qianbao’s founder turned himself over to authorities, Mr. Xu turned to fellow investors on WeChat, China’s popular social messaging platform, to commiserate. “I talked until 3 in the morning,” he said. “I was shocked.”

Now, he said, he must put the episode behind him. “I need to work and start over,” he said.

Some investors who lost their savings in Qianbao protested last week in the city of Nanjing, where the online investment platform had been based. Police acted swiftly, detaining the organizers of the demonstration and giving others warnings, according to a notice by the Nanjing police. Government censors appeared to have taken down some discussions about Qianbao on social media and removed some news articles about it.

China has been rife with investment frauds in the decades since its economic reopening led to a boom. But online versions have the potential to reach more people in a country with more than 700 million internet users, many of whom now conduct most of their financial transactions on smartphones.

Investors in online products are often drawn by promises of high rates of return and the idea that the investments are safer than China’s stock market, which has long had a reputation for casino-like uncertainty. But they are often unsophisticated investors who are unaware of the risks, experts warn.