Mr Law, 27, said he and other activists started the site from outside Hong Kong. A New York Times review of the digital route traffic to the site found it was hosted by servers in the United States.

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Mr Law said he had been walking back and forth with a representative at Wix since the Monday the site first went missing. At that point, the company informed him that there was a legal request for removal and that the website was in violation of the company’s terms of use. The company later sent the letter to Mr. Law from the Hong Kong Police, stating that the location was a national security threat.

The website contains a letter to Hong Kong residents who have fled the city asking them to unite in the pursuit of democracy in the city. She also calls for the repeal of the national security law, calls for police reform in Hong Kong and criticizes the Chinese Communist Party’s authoritarian rule.

“We strive for Hong Kong’s democratic transformation to achieve the freedom, autonomy and democracy that Hong Kong was promised,” reads one part of the letter. Visitors to the website can sign up for the document called “Hong Kong Charter 2021”.

Mr Law said the website does not encourage violence. “It doesn’t do anything that would be considered illegitimate in liberal countries, but the government can always cite the national security law” to decide a site is illegal, he said.

“So yes, we will face similar events in the future,” he added.

In January, Hong Kong’s largest wireless operator blocked access to a local Hong Kong website listing police officers’ personal information. The move heightened long-held fears that censorship rules as strict as China’s could be introduced in Hong Kong in the coming years.

This week, authorities said they would soon require residents to provide their real identities when purchasing cellular services. A similar system in China helped regulators end online anonymity and strengthened a force of Internet police officers to interview and sometimes imprison the most obvious.