Hong Kong's legislature on Thursday approved a sweeping electoral overhaul ordered by China to ensure that only Beijing loyalists rule the city.

The move by the Legislative Council, which is now largely devoid of pro-democracy legislators, caps Beijing's efforts to quash dissent and exclude anti-China elements from the government in Hong Kong, following sometimes violent anti-government protests in 2019.

"Hong Kong's political system comes under the purview of the central government," Secretary for Constitutional and Mainland Affairs Erick Tsang said in the legislature.

"The 3-11 resolution (on electoral reform) is constitutional, legal, reasonable and fair. It will bring Hong Kong back on the right track while fully implementing the principle of patriots administering Hong Kong," he said.

The development led the United States to denounce the Chinese government for continuing to undermine Hong Kong's democratic institutions.

The reform passed by the Hong Kong Legislative Council "severely constrains people in Hong Kong from meaningfully participating in their own governance and having their voices heard," Secretary of State Antony Blinken said.

All 40 pro-establishment Hong Kong lawmakers who spoke applauded the amendment.

"With the passage of the bill, Hong Kong can open a new era, and the society will be more stable," former security secretary-turned-lawmaker Regina Ip said before casting her vote.

The amendment bill, approved by the legislature with a 40-2 vote, will change the methods of choosing Hong Kong's leader and legislators, with a new review committee set up to vet all candidates seeking public office.

If questions arise about a candidate's qualifications on national security grounds, the Committee for Safeguarding National Security, established under the sweeping national security law, will issue an opinion on their qualifications based on a police assessment.

The amendments will reduce the number of directly elected seats in the Legislative Council from the current 35 to 20 while increasing overall membership in the legislature from 70 to 90.

A 1,200-strong Election Committee, tasked with picking the territory's chief executive, will be expanded to 1,500 members and also be responsible for nominating candidates for legislative elections and electing 40 members of the legislature.

"We are disappointed with the way the government is changing the electoral system because we can see that the representation of the people from Hong Kong in the Legislative Council or in the institution as a whole is much less than before," Democratic Party chairman Lo Kin-hei told reporters. "We don't think this is a good way to pacify the Hong Kong people."

He said the party, whose seven legislators staged a mass resignation along with eight others in November in protest against Beijing's disqualification of four colleagues, will decide after a general assembly in September if it will field any candidates for the legislative election.

The election for Election Committee members is set for Sept. 19. The next Legislative Council election, which was postponed from last September for a year on pandemic concerns, is set for Dec. 19, while the chief executive election is to be held on March 27, 2022.

Since the former British colony was returned to Chinese control in 1997, Beijing had guaranteed Hong Kong a "high degree of autonomy" under the principle of "one country, two systems" while highlighting the concept of "Hong Kong people administering Hong Kong."

But following the 2019 anti-government protests amid a growing influence of pro-democracy forces in the territory, the central government under President Xi Jinping called for Hong Kong's executive, legislative and judicial branches to be made up only of "patriots" who do not oppose the ruling Chinese Communist Party.