HONG KONG - Beijing's move to to directly enforce a national security law in the territory has fanned fears of further unrest as it elicited calls on social media for protests against the move, which has been viewed by some as an effective upending of the "one country two systems" principle.

Mr Jimmy Sham of the Civil Human Rights Front, the organiser behind some of the largest demonstrations in Hong Kong, condemned the decision, in particular, the move for Beijing to set up national security agencies in Hong Kong for enforcement purposes.

Describing it as "ridiculous", he urged people in a Facebook post to join the Front's activity when details are out.

"The Front was set up because of Article 23, Now that the national security law is at our doorstep, the Front will fight hard. We need everyone to rescue Hong Kong and we hope the numbers will cross two million," Mr Sham wrote in a rallying cry.

Earlier on Friday, online calls for an unauthorised march from the financial district in Central to Beijing's liaison office in Hong Kong in Sai Ying Pun were not heeded.

Associate Professor Allen Carlson of Cornell University's government department said that, when enacted, the law seems like "a definitive breach" of the "one country two systems" principle, which guarantees for 50 years a high degree of autonomy to Hong Kong after the territory was handed from the British back to China in 1997.

When it was pointed out that pro-establishment lawmakers have stressed that countries like the United States and Singapore also have national security legislation in place, Prof Carlson noted that comparisons between these states and China, particularly Hong Kong, "is facile at best".

This, as China's decision seriously undermines not only Hong Kong's legal framework, but is also in direct contrast to the manner in which it appears that the vast majority of those living in the city understand the basic social and political contract that exists between them and Beijing.

"In this sense, the proposed law is much less about the overall security of the People's Republic of China, and more about the way Beijing, under Chinese President Xi Jinping, is moving to redefine the relationship between Hong Kong and the rest of China," said Prof Carlson.

Associate Professor Alfred Wu of the Lee Kuan Yew School of Public Policy believes Chief Executive Carrie Lam would step down soon after the September Legislative Council election "because there's no longer a need for her to be around when the national security law is implemented".

"In her place, a candidate who is more loyal to Beijing will be installed; even the pro-democracy camp has criticised Lam for the pro-Beijing stance and ignoring local interests," Prof Wu noted.

China's announcement led Hong Kong stocks to plunge 5.6 per cent on Friday, its biggest drop since July 2015, while global equities slid on the news. MSCI's broadest index of Asia-Pacific shares outside Japan fell 2.7 per cent, European shares opened lower with blue-chip indexes in London, Paris and Frankfurt all down more than 1.5 per cent, and US stock futures dipped almost 1 per cent.

The American Chamber of Commerce in Hong Kong (AmCham) has urged clarification of Beijing's own new national security law on Hong Kong.

It warned in a statement that this could strike fear in the business community and foreigners living in the city.

In a brief session with the media in Hong Kong on Friday night, Mrs Lam was asked if she had given up on pushing out national security law on her own.

"I have been trying my best, but since we could not do it, I don't think we could do it in the foreseeable future, both in the executive as well as the legislature... I'm not giving up, but there's now another way of ensuring that Hong Kong will have the necessary legislation and the safeguards in place for national security."

Earlier in the day, she issued a statement promising to "complete the legislation as soon as possible".

"The decision does not amend the Basic Law or replace or repeal Article 23 of the Basic Law, which stipulates that the Hong Kong government has constitutional responsibilities and legal obligations to enact laws on its own to prohibit acts that endanger national security."

Mrs Lam added that the move will not affect the legitimate rights and freedoms enjoyed by Hong Kong residents under the law, nor the judiciary's independence.

The Hong Kong and Macau Affairs Office also weighed in on Friday, saying Beijing's move is "timely, necessary and very important", especially given that the law has not been enacted after 23 years.

Strong words were also exchanged on Thursday night, with the pan-democratic lawmakers and pro-government camp pointing accusing fingers at one another for China's move, which can bypass the city's Legislative Council (Legco).

The pan-dems have said Beijing's move spells the end of "one country two systems" principle.

Former Hong Kong leader Leung Chun Ying, who is a vice-chairman of the mainland's top advisory body, on Thursday blamed the opposition for Beijing's decision.

"Hong Kong will not be able to plug the loophole by way of protecting national security," he said in a video interview with mainland media, adding that the recent filibustering by the pan-dems that has delayed Bills, triggered this outcome.

Asked if the pan-dems had overplayed their hand, lawmaker Claudia Mo on Friday dismissed this.

"Of course not. If we don't fight, they'll do whatever they want. And if we fight, they use it as the excuse to do whatever they want," she told The Straits Times.

Associate Professor Sing Ming of Hong Kong University of Science and Technology disagreed with Mr Leung, telling ST that what the opposition lawmakers were trying to do was to make sure that the rule of law will continue to persist.

"They have been all along the way trying to protect the core values in Hong Kong and these core values are enshrined in the Basic Law and are the entitlements of the Hong Kong people. It is Beijing who is violating one after another, the promises made in the Basic Law," he said, adding that Mr Leung's argument is "unconvincing" as even the US president is subjected to checks and balances.

Under Article 23 of the Basic Law, which is Hong Kong's mini Constitution, the government is mandated to pass national security legislation to prohibit any act of treason, secession, sedition and subversion against the Central People's Government.

In 2003, when former Secretary for Security Regina Ip tried to invoke it, half a million Hong Kongers took to the streets in protest in fears that their freedoms would be eroded.