China has quickly moved to censor Hong Kong's internet and access users' data using a feared new national security law, but US tech giants offered some resistance citing rights concerns.
The online censorship plans were revealed in a 116-page government document released on Monday night that also unveiled expanded powers for police that will allow them to conduct warrantless raids and surveillance.
China imposed the law on semi-autonomous Hong Kong less than a week ago without revealing many details other than it would ban terrorism, subversion, secession and colluding with foreign forces.
Despite assurances that only a small number of people would be targeted by the law, the details that have since been released show it will be the most radical change in Hong Kong's freedoms and rights since Britain handed the city back to China in 1997.
"The Chinese Communist Party's destruction of free Hong Kong continues," US Secretary of State Mike Pompeo said on Monday.
Pompeo spoke out against what he branded a series of "Orwellian" moves to censor activists, schools and libraries.
"Until now, Hong Kong flourished because it allowed free thinking and free speech, under an independent rule of law. No more," Pompeo said.
Under its handover deal with the British, Beijing promised to guarantee until at least 2047 certain liberties and autonomy not seen on the authoritarian mainland.
But fears China's ruling Communist Party was steadily eroding those freedoms helped to build a powerful pro-democracy movement, which led to massive and often violent protests for seven months last year.
China has made no secret of its desire to use the law to crush the democracy movement, and to intimidate those who want to resist.
"The Hong Kong government will vigorously implement this law," Chief Executive Carrie Lam, the city's Beijing-appointed leader, told reporters on Tuesday.
"And I forewarn those radicals not to attempt to violate this law, or cross the red line, because the consequences of breaching this law are very serious."
With pro-democracy books quickly pulled out of libraries and schools, the government signalled in the long document released on Monday night that it would also expect full obedience online.
Police were granted powers to control and remove online information if there were "reasonable grounds" to suspect the data breaches the national security law.
Internet firms and service providers can be ordered to remove the information and seize their equipment, with fines and up to one year in jail if they refuse to comply.
The companies are also expected to provide identification records and decryption assistance.
However the biggest American tech companies offered some resistance.
Facebook, Google and Twitter said Monday they had put a hold on requests by Hong Kong's government or police force for information on users.
Facebook and its popular messaging service WhatsApp would continue to deny requests until it had conducted a review of the law that entailed "formal human rights due diligence and consultations with human rights experts," the company said in a statement.
"We believe freedom of expression is a fundamental human right and support the right of people to express themselves without fear for their safety or other repercussions," a Facebook spokesman said.
Twitter and Google told AFP that they too would not comply with information requests by Hong Kong authorities in the immediate future.
"Like many public interest organisations, civil society leaders and entities, and industry peers, we have grave concerns regarding both the developing process and the full intention of this law," Twitter said in response to an AFP inquiry.
In less than a week since the law was enacted, democracy activists and many ordinary people have frantically tried to scrub their online profiles of anything that China may deem incriminating.
Monday night's document also revealed that judicial oversight that previously governed police surveillance powers in Hong Kong had been eliminated when it comes to national security investigations.
Police officers will be able to conduct a search without a warrant if they deem a threat to national security is "urgent".
"The new rules are scary, as they grant powers to the police force that are normally guarded by the judiciary," barrister Anson Wong Yu-yat told the South China Morning Post.