China's parliament has passed an interpretation of Hong Kong's Basic Law that says lawmakers must swear allegiance to the city as part of China.
It's Beijing's most direct intervention in the territory's legal and political system since the 1997 handover.
The ruling is expected to bar two activist lawmakers from taking office in Hong Kong. The prospect of the ruling had sparked protests in the former British territory that returned to Chinese rule in 1997.
The official Xinhua news agency reported that China's parliament ruled at the end of a regular bimonthly session that the pair of pro-independence lawmakers could not assume their positions in Hong Kong's Legislative Council if they refused legal procedures when taking oath of office.
The intervention relates to Article 104 of the city's mini-constitution, which states that lawmakers must swear allegiance to Hong Kong as part of China when they take office.
The move was expected to enrage Hong Kong democracy activists further, a day after hundreds of demonstrators clashed with police in running battles around China's representative office in Hong Kong.
The scenes on Sunday night were reminiscent of pro-democracy protests in late 2014 that paralysed parts of the Asian financial centre and posed one of the greatest political challenges to the central government in Beijing in decades.