The 27-year-old Mr. Law said he and other activists set up the site from outside of Hong Kong. A New York Times check on the digital route taken by traffic to the site showed that it was hosted by servers in the United States.

Mr. Law said that he had gone back and forth with a representative at Wix since May 31, when the site first disappeared. At the time, the company told him that there was a legal takedown request and that the site was in violation of the company’s terms of service. Later, the company sent Mr. Law the letter from the Hong Kong police, which said the site was a threat to national security.

The site contains a letter, addressed to Hong Kongers who have fled the city, that calls for them to unite in striving for democracy in the city. It also calls for the repeal of the national security law, urges the reform of policing in Hong Kong and criticizes the authoritarian rule of China by the Chinese Communist Party. “We strive for Hong Kong’s democratic transformation, to realize the freedom, autonomy and democracy that were promised to Hong Kong,” reads a part of the letter. Visitors to the site can sign onto the document, which they call the “2021 Hong Kong Charter.”

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

“So yes indeed, we will face more similar events in the future,” he said.

In January, Hong Kong’s biggest mobile telecom companies severed access to a local Hong Kong website that listed the personal information of police officers. The move heightened long-held fears that censorship rules as strict as China’s could be ushered into Hong Kong in the coming years.

This week, authorities said they would soon require residents to use their real identity when purchasing cellular services. A similar system in China helped regulators end online anonymity and empowered a force of internet police officers who question and sometimes jail the most outspoken.

Although he was encouraged by Wix’s response, Mr. Tsui said that resistance from tech companies to police orders could push the authorities to take matters into their own hands, and, as in China, start blocking more websites directly.