Hong Kong's leader said on Monday he would "fully implement" a ruling by Beijing which effectively bars two elected pro-independence lawmakers from the city's legislature after after they deliberately misread their oaths of office.

It comes as fears grow that China is tightening its grip on the semi-autonomous city with concerns that Hong Kong's rule of law and court system are now under threat.

Beijing's intervention pre-empts a decision by Hong Kong's High Court into whether rebel lawmakers Baggio Leung and Yau Wai-ching should be disqualified from taking up their seats. That court decision is still pending.

An oath that did not conform to Hong Kong's law "should be determined to be invalid, and cannot be retaken", the Communist-controlled National People's Congress (NPC) in Beijing said in a rare interpretation of the city's constitution.

Hong Kong's leader Leung Chun-ying told reporters that he and the city government would "implement the interpretation fully".

"Any words or deeds that deliberately contravene (the interpretation's) requirements, defy the prescribed oath-taking procedures, or even use the opportunity to insult the country and the Chinese people and advocate cessation, must be stopped in accordance with the law," Leung said.

Hong Kong is a former British colony which returned to Chinese rule in 1997 under a promise of "one country, two systems" for 50 years.

But there are deep concerns those liberties, enshrined in the city's constitution known as the Basic Law, are at risk, and mass protests two years ago demanded greater democracy.

Yau and Baggio included expletives and derogatory terms in their oaths of office last month, and draped themselves in "Hong Kong is not China" flags.

They sought a second chance to take their oaths, but both the Hong Kong government and Beijing have stepped in to prevent that.

Beijing sees any talk of independence as treasonous and the official Xinhua news agency quoted a spokesman for the government's Hong Kong and Macao Affairs Office welcoming the ruling.

"The interpretation demonstrates the central government's firm determination and will in opposing 'Hong Kong independence,'" the spokesperson said, adding that it "complies with the common aspiration of the entire Chinese people, including the compatriots in Hong Kong".

Angry protests

Hong Kong police used pepper spray Sunday to drive back hundreds of protesters angry at China's decision to intervene in the row.

In chaotic scenes reminiscent of the demonstrations of 2014, protesters charged metal fences set up by police outside China's liaison office in the city.

Monday's announcement was the fifth time since the handover that China has interpreted the Basic Law.

Li Fei, the chairman of the Basic Law Committee of the NPC Standing Committee, told reporters: "These Hong Kong independence figures are going against the country and splitting the country. How can they uphold Hong Kong's Basic Law?"

"I hope people can see the real face of these people," he added.

Both those promoting independence and those arguing for self-determination were supporting separatism, he said -- anathema to China's ruling party.

Yau and Leung are among several newly elected lawmakers advocating self-determination or independence who won seats in citywide polls in September.

The pair have yet to be sworn into Hong Kong's main lawmaking body, the Legislative Council (Legco), after their first oath attempt last month was declared invalid.