China warned Thursday were a "blatant challenge" to its authority over the city.
Fifteen legislators were set to quit the chamber in protest at the Beijing-sanctioned ousting of four colleagues, leaving the assembly a muted gathering of government loyalists.
The resignations come with the city's beleaguered pro-democracy movement and avenues of dissent already under sustained attack since Beijing imposed a sweeping national security law this year.
Half of the group had made good on their pledge by Thursday afternoon, which sparked a furious response from Beijing's Hong Kong and Macau Affairs Office.
"It once again showed their stubborn confrontation against the central government and a blatant challenge to the power of the central government. We severely condemn this," a statement said.
"We have to tell these opposition lawmakers, that if they want to use this to advocate a radical fight, and beg for foreign forces to interfere, and once again drag Hong Kong into chaos, that's a wrong calculation."
Inside the chamber, government loyalists discussed a transport bill, but without any of the rambunctious debate that has been the mark of Hong Kong's semi-democracy in recent years.
"Hong Kongers - prepare for a long, long time where there is only one voice in society," pro-democracy politician Lam Cheuk-ting told reporters outside.
"If you are a dissident, get ready for even more pressure."
Hong Kong Chief Executive Carrie Lam, a Beijing appointee, on Wednesday was granted the power to turf out any legislator who she deems insufficiently patriotic, without recourse to the city's courts.
She immediately made use of those powers, kicking out four politicians she said were a threat to national security, and sparking criticism both at home and abroad, with the United States threatening further sanctions on regime figures.
Chris Patten, the city's last colonial governor, said the move demonstrated Beijing's "total hostility to democratic accountability, and those who wish to stand up for it".
China's foreign ministry spokesman Wang Wenbin rejected the international criticism.
Empty seats of pro-democracy legislators, top, are seen at Legislative Chamber in Hong Kong, Thursday, Nov. 12, 2020. Source: AP
"We urge the relevant people to strictly abide by the basic norms of international law and international relations, stop any form of interference into China's internal affairs, of which Hong Kong affairs are a part," he said.
The exodus of opposition politicians neuters one of the last forums for dissent in Hong Kong, as its once-boisterous media reels under a crackdown unleashed by the national security law and Beijing loyalists target the legal system that has underpinned the city's success as a finance hub.
Restrictions on gatherings, partly because of the coronavirus, have also put the lid on the kind of huge rallies that roiled the city last year.
Millions took to the streets in largely peaceful protests over a lack of political accountability and what demonstrators saw as overbearing policing.
Violence and vandalism erupted at some demonstrations, and more than 10,000 people were arrested.
'Final nail in coffin'
Hong Kong's leader is chosen by pro-Beijing committees, but half of the legislature's 70 seats are directly elected, offering the city's 7.5 million residents a rare chance to have their voices heard at the ballot box.
Scuffles and protests would routinely break out in the chamber, with the out-gunned pro-democracy minority often resorting to filibustering and other tactics to try to halt bills they oppose.
The expulsions and resignations will leave just two legislators outside the pro-Beijing camp, both of them unaligned with either bloc.
"It seems that the control of Hong Kong has now been exercised by the Chinese Communist Party authority in Beijing," political analyst Willy Lam said, adding the basic rights enshrined when Britain handed the city back to China in 1997 were "seriously jeopardised".
The move by Beijing "has put the final nail in the coffin," Claudia Mo, one of the politicians who resigned, told AFP. "It's rule by decree."
"What's the point of going to work every morning thinking 'am I going to be kicked out'?"
