Facebook, Google and Twitter have suspended processing government requests for user data in Hong Kong, following China's establishment of a sweeping new national security law for the semi-autonomous city. 

Facebook, which also owns WhatsApp and Instagram, said in a statement it was pausing reviews for all of its services "pending further assessment of the National Security Law." 

Google, a unit of Alphabet, and Twitter said they suspended their reviews of data requests from Hong Kong authorities immediately after the law went into effect last week. 

Twitter cited "grave concerns" about the law's implications. 

Google said it would continue reviewing Hong Kong government requests for removals of user-generated content from its services. Twitter declined to comment, while Facebook did not respond to a request for comment. 

Social networks often apply localised restrictions to posts that violate local laws but not their own rules for acceptable speech. 

Facebook restricted 394 such pieces of content in Hong Kong in the second half of 2019, up from eight in the first half of the year, according to its transparency report. 

Tech companies have long operated freely in Hong Kong, a financial hub where internet access has been unaffected by the firewall imposed in mainland China, which blocks Google, Twitter and Facebook. 

In addition to the announcements by the UStech giants, TikTok, the short-form video app owned by China-based ByteDance, said it would pull out of the Hong Kong market within days. 

TikTok was designed so it could not be accessed by mainland China, part of a strategy to appeal to a more global audience.

Hong Kong is a small, loss-making market for the company, one source familiar with the matter said. 

China's parliament passed the national security legislation last week, setting the stage for the most radical changes to the former British colony's way of life since it returned to Chinese rule 23 years ago. 

Hong Kong last night published more details about how the new law will strengthen police powers over the internet, including the ability to ask publishers to remove information deemed a threat to national security, refusal of which could result in a fine or jail. 

Asked about the moves by the US tech firms and prospects for media freedom, Hong Kong Chief Executive Carrie Lam told a news conference on Tuesday: "Ultimately, time and facts will tell that this law will not undermine human rights and freedoms."

Apple said it does not receive requests for user content directly from the Hong Kong government. Instead, it requires authorities there to submit requests under a mutual US-Hong Kong legal assistance treaty. 

The US Department of Justice receives the requests and reviews them for "legal conformance," Apple said. 

"We're assessing the new law, which went into effect less than a week ago, and we have not received any content requests since the law went into effect," Apple said in a statement. 

Some Hong Kong residents have said they are reviewing their posts on social media related to pro-democracy protests and the security law, and deleting ones they thought would be viewed as sensitive. 

Messaging app Signal, which promises end-to-end encryption, has seen a surge in sign-ups by Hong Kong residents in recent days. 

"We'd announce that we're stopping too, but we never started turning over user data to HK police. Also, we don't have user data to turn over," it said on Twitter.