A Chinese city may have used a Covid app to block protesters, drawing an outcry.

Since the pandemic’s early days, China has used mobile apps to identify and isolate people who might be spreading Covid-19. Now, a central Chinese city may have shown a far more troubling use of that data: stopping would-be protesters.

Dozens of people from across China had set off for the city of Zhengzhou days ago to protest the freezing of their savings amid an investigation into several regional banks. But when they arrived in the city, many found that the so-called health codes on their phones had turned from green — meaning good — to red, a designation that would prevent them from moving freely.

Tom Zhang, the owner of a textile business in the eastern province of Zhejiang, said this happened to him when he was on a train headed to Zhengzhou on Sunday, despite coming from a town where there had been no Covid cases. Upon arriving in the city, Mr. Zhang said, he was stopped by the police and told that his red code — usually suggesting an infection or close contact — indicated he posed public health risk. He said the Zhengzhou police had held him in a local library for around 12 hours.

“The red code was definitely used to limit us depositors,” Mr. Zhang said in a phone interview. “It was a complete absurdity.”

Mr. Zhang was part of a group of hundreds of depositors in several rural banks who had planned to lodge a complaint with the Henan Province bank regulator on Monday after having been unable to withdraw their money for months. (Mr. Zhang said his account held about $440,000 in savings.) Several other people from this group also told The New York Times that their health codes had changed en route to Zhengzhou.

Many of the petitioners posted their red health codes on Chinese social media sites, suggesting that the inexplicable change was no coincidence. Their complaints quickly fueled widespread outrage and prompted a slew of questions, even drawing an unusual rebuke from establishment commentators, who asked if the local authorities had abused their power.

Hu Xijin, a former editor of the ruling Communist Party’s Global Times newspaper, warned that the use of the health code for purposes other than epidemic control “damages the authority” of the monitoring system and would chip away at the public’s support for it. His post on Weibo, a Twitter-like social media platform, on Monday became a hashtag that was among the most-searched earlier this week, drawing 280 million views.

“Health code can’t be abused,” China Comment, a monthly magazine managed by the official Xinhua News Agency, wrote in a commentary. The magazine said the Henan authorities had to explain how the codes changed, whose decision it had been to use the codes this way and what procedures had been followed. It also said standards should be strengthened for the use of health codes to guard against their abuse by “willful power.”

A worker with the Zhengzhou government hotline for collecting residents’ complaints told The Times that the hotline had received many complaints about the abnormal red code and that authorities were looking into the claims. The Henan Provincial Health Commission told Chinese media that it was investigating the complaints from the depositors about their red codes.

While he was being held by the police, Mr. Zhang said, employees from the bank regulator told him that the investigation into the banks could take two years. With little option for recourse, Mr. Zhang said, he agreed when police officers from his hometown arrived and offered to escort him out of Henan.

As he was making his way to the airport, his health code suddenly turned green.