But on Monday night, social media app Clubhouse appeared to have been blocked in China just days after it became the go-to app for uncensored conversations on a host of sensitive issues banned on other platforms.

By Monday evening, many Clubhouse users in mainland China reported that when they tried to log onto the app, they received a red error message showing "a secure connection to the server cannot be made."

On Tuesday, the hashtag "Clubhouse" was also censored on Chinese social media platform Weibo, where it had been trending. People with mainland phone numbers reported no longer being able to receive text messages from Clubhouse, in effect blocking them from joining as invitation and verification codes are sent to a mobile phone to register a new account.

On Clubhouse, several chat rooms soon sprang up to discuss the blocking of the app. They were joined by hundreds of users, including some who said they were based in mainland China. Greatfire.org, a group which monitors internet censorship in China, also confirmed that the app had been blocked.

The ban, however, came as little surprise. With its political discussions drawing so much interest from mainland China, many users and observers expected it was only a matter of time before the app was blocked. While the censorship might deter new users, it is unclear how many existing users will be kept off the platform.

Susan Liang, a 31-year-old from Shenzhen, said she would continue to join Clubhouse chats on sensitive topics via a VPN because she didn't want to give up the frank and open discussions.

"It is too rare an opportunity. Everyone has lived under the Great Firewall for so long, but on this platform, we can talk about anything," she said. "It's like someone drowning, and can finally breathe in a large gulp of air."

But Liang expects some other users might be discouraged by having to use a VPN, as that technology has been increasingly targeted by Chinese government crackdowns. Any VPN not approved by the government is illegal.

Benjamin Ismail, an expert with Apple Censorship a project run by GreatFire.org said some users would be discouraged by the block but "it might not kill the app immediately" in China.

Popular political chat rooms

While the app first became popular in China among tech industry circles, its political chat rooms quickly drew newcomers eager for release from the tight censorship at home. As it grew in popularity, many Chinese also joined to discuss topics such as culture, lifestyle and celebrity gossip. But the space for free, inclusive political discussions was one of the rarest qualities of the app for Chinese-speaking communities.

One chat room hosted by Taiwan-based blogger Zola was running non-stop for almost 120 hours, joined by Chinese speakers in different time zones.

Another popular chat room invited young people from both sides of the Taiwan Strait to share their views and personal stories. The discussions started with lighthearted subjects but soon turned to politics, with users comparing the political systems of China and Taiwan and debating the prospects of unification.

Started Friday evening, the room soon attracted hundreds of people, and reached the upper limit of 5,000 listeners around midnight, according to Tan.

Several Han Chinese from Xinjiang also shared their experience of the security crackdown. A number of overseas Chinese broke down in tears describing the sense of guilt they feel over the alleged human rights abuses in Xinjiang, while others defended Beijing's policies, and questioned accounts of abuse from the region.

Other users and outside observers expressed skepticism over how representative the groups engaging in these political discussions are of broad Chinese public opinion, pointing to the self-selecting nature of the participants, as well as the barriers to using Clubhouse itself which prevent it from being a completely public app.

"Political topics on the platform are not discussed as rationally as other topics like technology or culture," the paper said.

But even before the app was blocked, there were potential security concerns for users within mainland China. Accounts are also tied to users' mobile phone numbers, which in China are registered under owners' real names. Furthermore, it would be a relatively simple task for the Chinese authorities to infiltrate open chat groups on issues such as Xinjiang and record what is being said for future use.

Badiucao, a Chinese dissident artist based in Australia, said some Chinese users, especially those within China, might not have realized the potential risk before speaking out critically against the government's policies, even semi-anonymously.

"If they were typing their opinions out, they might have the time to think it over," he said. "But when they spoke in these real-time chat rooms, they might not be able to hold their tongue."

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For a brief period, there was a platform for sensitive political debate in China. Then censors shut it down - CNN