At midnight on Tuesday, the Great Firewall of China, the vast apparatus that limits the country’s internet, appeared to descend on Hong Kong.

Unveiling expanded police powers as part of a contentious new national security law, the Hong Kong government enabled police to censor online speech and force internet service providers to hand over user information and shut down platforms.

Many residents, already anxious since the law took effect last week, rushed to erase their digital footprint of any signs of dissent or support for the last year of protests. Charles Mok, a pro-democracy lawmaker who represents the technology sector, tweeted: “We are already behind the de facto firewall.”

Hong Kong is facing a dramatic decline of one of its most important advantages – a free and open internet – a defining trait that sets it apart from mainland China where Facebook, Twitter, Google and most major foreign news sites are blocked.

The prospect of Beijing-style internet controls – where residents are not just restricted but monitored and punished for what they post online while companies are forced to censor their platforms – is worrying for citizens, activists and businesses in Hong Kong.

The law gives authorities the power to demand individuals and service providers remove content, or access to content deemed threatening to national security. Noncompliance can result in fines and imprisonment for company staff or individuals. Police investigating national security cases can surveil communications and confiscate electronic devices.

“The law seems to be building up the Great Firewall locally in Hong Kong. Personal freedom on the internet will be eliminated,” said Charles Low, the chairman of the Hong Kong chapter of the Internet Society. “If you say something wrong they can request the service provider to give your IP address or mobile number so they can grab you.”

After the new measures were announced late on Monday, Facebook, Microsoft, WhatsApp, Google, Twitter, Telegram and others said they would not process information requests from the government until they had reviewed the law. TikTok, owned by the Chinese company ByteDance, said it was leaving Hong Kong altogether.

“We used to be an internet and telecom hub in the region. Companies moved services from the mainland to Hong Kong and now Hong Kong has become like China, so they will leave,” said Mok.

Protesters who have relied on digital tools over the last year to mobilise demonstrations now find those same platforms could be used against them. Political groups have already disbanded and formerly outspoken activists have quietly left social media, while others have deleted old comments.

“We had freedom before and now it is being taken away. It is extremely painful for me to experience that,” said Glacier Kwong, a digital rights activist. “There will be a loss of information available to the public because people are afraid to speak up. They are controlling the discourse, how people can think about things and what they can think about. It’s very dangerous.”

Experts say it is precisely because Hongkongers used digital tools so effectively against the Beijing-backed government that authorities are now targeting the online space. The movement that erupted last year managed to mobilise itself without leaders through platforms such as the LIHKG forum and messaging app Telegram – with a level of organisation that Beijing has tried to point to as evidence to claim the demonstrations are coordinated by foreign forces.

“It comes back to how effectively Hong Kong used the internet. To crack down on the protests you’ve got to take away that tool,” said Elise Thomas, a researcher at the Australian Strategic Policy Institute focusing on internet freedom.

Experts point out that China’s Great Firewall – which allows the government to inspect data as well as block IP addresses and domain names – could not be immediately replicated in Hong Kong, home to several private internet service providers and internet exchanges.

“It will take at least a few years to build up the wall,” said Low, adding that what is more likely is a partial blackout, cutting off access to certain sites such as LIHKG or Telegram.

Others worry the measures could go even further than in mainland China. The law covers not only permanent residents and foreigners within Hong Kong, but anyone seen as violating the law, regardless of where they are in the world.

The security law may also add to the Balkanisation of the internet, with countries having their own fenced-off versions, and major international tech companies will be under pressure not to contribute to that.

“If we’ve been talking about the ‘splinternet’ and the idea of these cracks skewing through the ice, the national security law is really an ice pick,” said Thomas. “It’s driven a nail in, it is driving those cracks much deeper and faster than before.”

One goal of the restrictions in Hong Kong could simply be making access to certain platforms and technology difficult enough that regular citizens will not bother – a strategy analysts say authorities also use on the mainland.

But Hongkongers, accustomed to decades of unrestricted access to information, may not be so easily deterred. Since Beijing announced its plan in late May to enforce the security law, searches and purchases of virtual private networks (VPNs) and proxies to hide IP addresses have soared.

Many have migrated from Telegram to the encrypted messaging app Signal, and some residents have turned to sim cards from providers in other countries. Kwong says it is not just young protesters who are taking action – her parents recently moved their family group chat to Signal.

“People are indeed kind of panicked and trying to install VPNs and have no idea what it can and cannot help,” said Low, noting that volunteers have been holding workshops to teach residents how to use such tools and how to better protect themselves.

“I have faith in Hong Kong people. They will not forget about the freedom we once had.”