NEWARK — Zhou Fengsuo, a leader of the 1989 Tiananmen Square uprising, hoped to leave Chinese censorship behind when he fled to the United States and became a U.S. citizen. But Chinese censors have caught up with him, through the social-networking service WeChat.

The mobile app, born in China and used by Mandarin speakers around the globe, has long blocked Zhou’s friends in China from seeing the political posts he shares from the WeChat account he created in the United States, Zhou says. Then about a year ago, the problem got worse, he says — friends with both U.S. and Chinese accounts said they couldn’t see his timeline posts, whether the material was political or mundane.

On a recent morning at Zhou’s third-floor walk-up apartment, he and his colleague, Ouyang Ruoyu, took out their phones to demonstrate the blockade. On Zhou’s phone, his recent WeChat posts were visible — pictures of fall foliage in the Catskills, a message celebrating the memory of the dissident and Nobel Peace Prize laureate Liu Xiaobo. But viewed from the U.S.-registered account on Ouyang’s phone, the space beneath Zhou’s profile photo was an empty white screen.

Two of Zhou’s other friends living in the United States, also using accounts created in the United States, said they couldn’t see Zhou’s posts either.

Seeing this kind of censorship leak into the United States is why Zhou says he supports the Trump administration’s push to ban WeChat.

“WeChat is a prison. It’s a gulag,” said Zhou, who runs the nonprofit group Humanitarian China. “For the United States, it’s a Trojan horse to influence society at every level. … That’s why it must be banned here.”

A dozen WeChat users in the United States and Canada shared censorship stories with The Washington Post, ticking off cases of messages that they sent from their North American phones disappearing before reaching friends — at times when those friends were also located in the United States and Canada. Some users also spoke about being unable to log into their accounts after sharing information critical of China.

Several of these users said they, too, support the White House’s aim of banning the app. Others said they don’t support a ban, but want the United States to pressure WeChat’s owner, the Chinese tech giant Tencent, to stop censoring content.

“Sue it, punish it, fine it,” said Yang Jianli, a survivor of the Tiananmen Square massacre who now runs a nonprofit organization in Washington. The group, Citizen Power Initiatives for China, is attempting to organize a class-action lawsuit against Tencent, recruiting U.S.-based plaintiffs who have experienced censorship or other problems on WeChat.

In an emailed statement, Tencent spokesman Sean Durkin said the company “operates in a complex regulatory environment, both in China and elsewhere.”

A “core” tenet of the global company, he said, “is that we comply with local laws and regulations in the markets where we operate.”

WeChat has millions of users in the United States, who use it to keep in touch with family in China, where most Western communication apps, including Facebook, WhatsApp and Telegram, are banned. WeChat is known as Weixin inside China, where it is an enormously popular tool for connecting with friends, ordering food, reading news and shopping online.

Durkin said Tencent considers WeChat and Weixin to be “sister apps” that are “separate but interoperable,” with “each addressing different users groups and offering different content and features,” as well as being subject to “different regulatory environments.”

The Trump administration tried to ban WeChat from U.S. app stores in September, saying it posed threats to national security because it collects “vast swaths” of data on Americans and other users, and offers the Chinese Communist Party an avenue for censoring or distorting information.

But in September, a federal judge in San Francisco temporarily halted the ban in response to a lawsuit from WeChat users in the United States, saying the plaintiffs had raised “serious questions” about a ban harming their First Amendment rights.

“Certainly the government’s overarching national-security interest is significant. But on this record — while the government has established that China’s activities raise significant national security concerns — it has put in scant little evidence that its effective ban of WeChat for all U.S. users addresses those concerns,” U.S. Magistrate Judge Laurel Beeler wrote in a Sept. 19 order granting a preliminary injunction while the case proceeds.

One of the plaintiffs, Elaine Peng, a U.S. citizen in California who runs a nonprofit providing mental health care, told the court that she relies on WeChat to communicate with elderly Chinese American patients and their families. “Since many of the Chinese community members we serve are not fluent in English, WeChat is the only online tool that they rely on,” Peng said in a declaration filed in court. WeChat has 2.3 million weekly active users in the U.S., according to analytics provider App Annie.

An appeals-court hearing is scheduled for Jan. 14 to consider the government’s motion to lift the preliminary injunction. President-elect Joe Biden’s transition team didn’t respond to a request for comment on the ban effort.

George Shen, a Chinese American technology executive in the Boston area, said he understands the judge’s concerns, but thinks the court should consider that WeChat “restricts freedom, rights and speech in this country.”

Shen said he has experienced censorship several times on the WeChat accounts he created in the United States. First, a photo he posted of Liu, the late dissident who was awarded the Nobel Peace Prize in 2010 while serving a prison sentence for “inciting subversion,” was deleted from his timeline, Shen said. Then months later, in March 2019, his account was blocked with no explanation — Shen couldn’t log in for about a year. Soon after he created an online petition, calling for Tencent to “stop illegal censorship … or face sanctions.”

Shen created two additional U.S. accounts, and used them in June 2019 to share photos of Hong Kongers commemorating the victims of the Tiananmen Square massacre. “Both accounts, within a couple of hours, were immediately blocked,” he said, adding that he was unable to log in for a week or two.

Eventually he regained access to all of his accounts, but now nothing he shares from his original account — not even mundane, nonpolitical information — is visible to his friends in China, said Shen, who wrote a blog post recommending ways to avoid WeChat when communicating with people in China.

Chinese authorities require Tencent to heavily censor the app inside China. Posts about Chinese politics — and many other topics — disappear when they are sent to or from a China-registered account. Chinese authorities have used the app to monitor political dissidents and other critics, some of whom have been detained by police or sentenced to prison for their communications.

That censorship doesn’t remain in China, however. If a Chinese student or worker moves abroad and continues using an account created in China, the censorship will remain, according to Jeffrey Knockel, a research associate at the University of Toronto’s Citizen Lab, which studies information technology and human rights.

“Even if you move to the U.S. and switch your account to a U.S. number and U.S. device, you are still under Chinese political censorship,” he said, adding that many people like to keep their Chinese accounts to retain their contact lists and digital-payment details.

Tencent spokesman Durkin confirmed that an account created in China will always be treated as a Chinese Weixin account, even if the user moves abroad and accesses it from an overseas device.

“If a WeChat user sends a message to a friend using Weixin, China law applies to the Weixin user and certain content may be blocked,” he said in his emailed statement.

In a 2016 report, Citizen Lab said the number of users potentially affected by this cross-border censorship was “vast,” including “students studying abroad, tourists, business travelers, academics attending international conferences, and anyone who has recently emigrated out of China.”

Knockel said Citizen Lab hasn’t documented any automated political censorship of communications traveling only between WeChat accounts created outside of China. But Zhou’s case shows that some U.S.-registered accounts are indeed blocked for other U.S.-registered users. Durkin declined to comment on Zhou or other individual cases.

Last year, Citizen Lab researchers reported another disturbing phenomenon: WeChat was subjecting overseas accounts to surveillance to train algorithms used to censor information in China.

“We show that files and images shared by WeChat users with accounts outside of China are subject to political surveillance, and this content is used to train and build up the censorship system that WeChat uses to censor China-registered users,” Citizen Lab researchers wrote.

If the United States had stronger data protection laws, Tencent might have had to disclose this surveillance to users, Knockel said. “If that sort of transparency were necessary and people understood the risks of using the app, then maybe we wouldn’t have to worry about whether to ban it,” he said.

Asked about the report, Tencent said: “With regard to the suggestion that we engage in content surveillance of international users, we can confirm that all content shared among international users of WeChat is private.”

Zhou left China for the United States in 1995, after serving a prison sentence for his leadership role in the Tiananmen protests. He went to business school at the University of Chicago, spent 19 years working in finance and then gave up gainful employment to work for Humanitarian China, which he co-founded in 2007 to provide aid to families of political prisoners in China.

He said he created a WeChat account in the United States about six years ago. It was a useful way to contact people back home, but he experienced censorship early on, hearing from friends in China that they couldn’t see his political posts.

Then about a year ago, friends with U.S. accounts started telling him they couldn’t see his timeline. His colleague at Humanitarian China, Ouyang Ruoyu, has two accounts — one that he created in China and another that he created after moving to the United States because Tencent kept suspending his Chinese account over his criticism of China, he said. On both accounts, Zhou’s timeline is blank, Ouyang demonstrated for The Post, toggling between his accounts on his U.S. phone.

Ouyang came to the United States as an asylum seeker in 2019, after running into trouble with Chinese authorities over his and his father, Ouyang Yi’s, political activism, he said. He kept using the WeChat account he created in China, logging into it via a username and password on his U.S. phone, because he wanted to keep in touch with his contact list. But at times his friends can’t see what he’s sharing.

In early December, Ouyang wrote a post on his Chinese account expressing support for Zhang Zhan, a Chinese journalist sentenced to four years in prison for her coverage of the Wuhan coronavirus outbreak. On Ouyang’s phone, the post successfully appeared on the timeline of his Chinese account.

But several days later, a friend in China said he couldn’t see the message. And when Ouyang logged into his own U.S. account to check whether he could see the post on his Chinese account, he couldn’t.

“I just read ‘1984.’ There is a sign, ‘Big Brother is watching you.’ That is what I feel,” Ouyang said about WeChat, adding that he supports a U.S. ban.

Jiabao “Jack” Ji, a Chinese law student at the University of Wisconsin at Madison, also maintains two WeChat accounts. He mostly uses his original account, which he registered in China, but he also created one in the United States.

Ji said he treats the censorship almost like a game, drumming up new ways to try to trick the WeChat algorithms that block content.

In summer 2019, when Ji was trying to share photos on his Chinese account of the pro-democracy protests in Hong Kong, his posts weren’t visible to others.

“If you want to post a picture of a kid in Hong Kong who got shot by police, the algorithm doesn’t allow you to,” he said. “You have to do a lot of tweaking to un-censor it.”

From his Madison apartment, he found a workaround, realizing that the photos would be visible if he posted them upside down. Later, when that technique stopped working, he started using Photoshop to draw random yellow lines on sensitive pictures, which allowed the photos to escape censorship.

Ji said he continues using WeChat “for sheer convenience,” to keep in touch with Chinese friends. He said human rights activists in China often use the encrypted messaging app Signal, one of the few Western apps that isn’t blocked, or Telegram, another encrypted app that Chinese users can access through a virtual private network.

But “if you want to connect to normal people in China, you have to have a WeChat account,” Ji said.

Asked about the proposed ban, Ji initially said he supported it, because it would force Chinese speakers to find a different communication tool that the Chinese authorities have less ability to control. Later, he said he had “mixed feelings” because as a libertarian, he has concerns about the U.S. government using its power to ban a messaging tool.

A short drive from Princeton, N.J., Teng Biao and his family have grown accustomed to grappling with WeChat censorship.

Early last year, Teng opened his U.S.-registered account to praise Li Wenliang, a Chinese doctor silenced by authorities for sounding an early alarm about coronavirus. But Teng’s family member, who lives under the same New Jersey roof, couldn’t see the post on his China-registered account, which he logs into on his U.S. phone.

And when Teng’s wife, Lynn Wang, tried to post an item to her China-registered WeChat timeline in December, she had to delete several politically sensitive words and names before anyone could see the item.

Teng, a dissident who fled China after clashing with the authorities over his human rights work, said he often censors himself on WeChat, avoiding political posts and mostly sticking to personal photos and news so his friends back home “might know I am still alive.”

He agrees that banning WeChat would “bring a lot of inconvenience” to Chinese speakers. But ultimately Teng said he supports the idea.

“I think WeChat should be banned because it is a censorship tool and also a propaganda and misinformation tool,” he said. “WeChat is controlled by the Chinese authorities. It’s not like another Twitter or Facebook.”

Eva Dou contributed to this report.