On Wednesday Beijing passed a resolution allowing the city's government to disqualify politicians deemed a threat to national security.
Shortly afterwards the opposition lawmakers said they would leave the city legislature in solidarity.
For the first time since Hong Kong was handed back to China in 1997 the body has almost no dissenting voices.
BBC China correspondent Stephen McDonnell says the legislature was already stacked in favour of the pro-Beijing-camp.
The territory's leader, Chief Executive Carrie Lam, is pro-Beijing and is supported by the central government there.
The move is being seen as the latest attempt by China to restrict Hong Kong's freedoms, something Beijing denies.