Hong Kong's pro-democracy legislators have resigned en masse after the city's government moved to disqualify four of them.

The pro-democracy camp announced the decision in a news conference, hours after the Hong Kong government said it would disqualify Alvin Yeung, Dennis Kwok, Kwok Ka-ki and Kenneth Leung from the legislature.

The disqualifications came after China's National People's Congress Standing Committee, which held meetings on Tuesday and Wednesday, passed a resolution saying those who support Hong Kong's independence or refuse to acknowledge China's sovereignty over the city, as well as commit acts that threaten national security or ask external forces to interfere in its affairs, should be disqualified.

"Today we will resign from our positions, because our partners, our colleagues, have been disqualified by the central government's ruthless move," Wu Chi-wai, convener of the pro-democracy camp, said at the news conference.

"We are facing a lot of difficulties in the coming future for the fight of democracy, but we will never, never give up," he said.

Mr Wu said the pro-democracy legislators will hand in their resignation letters on Thursday.

During the news conference, pro-democracy members chanted while holding hands.

"This is an actual act by Beijing... to sound the death knell of Hong Kong's democracy fight, because they would think that from now on anyone they found to be politically incorrect or unpatriotic or are simply not likable to look at, they could just oust you using any means," said pro-democracy legislator Claudia Mo.

Beijing has in recent months moved to clamp down on opposition voices in Hong Kong with the imposition of a national security law, after months of anti-government protests last year rocked the city.

"In terms of legality and constitutionality, obviously from our point of view this is clearly in breach of the basic law and our rights to participate in public affairs, and a failure to observe due process," said Mr Kwok, one of the disqualified legislators, referring to Hong Kong's mini-constitution.

Hong Kong's leader, Carrie Lam, said in a news conference on Wednesday that legislators must act properly, and that the city needs a legislature comprised of patriots.

"We cannot allow members of the Legislative Council who have been judged in accordance with the law to be unable to fulfil the requirements and prerequisites for serving on the Legislative Council to continue to operate in the Legislative Council," Ms Lam said.

Earlier in the year, the four now-disqualified pro-democracy politicians were barred from running for legislative elections originally scheduled for September, prior to the government stating that it would postpone the elections by a year due to coronavirus. They were disqualified over their calls for foreign governments to impose sanctions on Hong Kong and Beijing.

They later remained in their posts following the postponement of the elections.

The postponement was criticised by the pro-democracy camp as an attempt to block them from taking a majority of seats in the legislature, after they had held an unofficial pro-democracy primary participated in by over 600,000 voters to decide which candidates to field.

Chinese foreign ministry spokesperson Wang Wenbin said that the move to disqualify the legislators was necessary to maintain rule of law and constitutional order in Hong Kong.

"We firmly support the (Hong Kong) government in performing its duties in accordance with the Standing Committee's decision," Mr Wang said at a regular news conference.

Beijing's imposition of the national security law on Hong Kong in June and the ensuing crackdown on opposition voices brought condemnation from many western democracies, with several countries suspending their extradition treaties with Hong Kong.

Beijing has rejected all such criticism and lashed out at what it calls gross foreign interference in Chinese politics.

"I want to emphasise that Hong Kong is a special administrative region of China," Mr Wang said, reiterating Beijing's position. "The issue of qualification of Hong Kong Legislative Council members is purely China's internal affairs. No countries have the right to make irresponsible remarks or intervene."