An oath that did not conform to Hong Kong's law "should be determined to be invalid, and cannot be retaken", the Communist-controlled National People's Congress (NPC) in Beijing said in a rare interpretation of the semi-autonomous city's constitution.
Legislators had to be "sincere and solemn" when taking their oaths, and had to pledge "completely and solemnly" to uphold the Basic Law of Hong Kong as part of the People's Republic of China, the NPC declared at the Great Hall of the People on Tiananmen Square.
There are deep concerns those liberties, enshrined in the city's constitution known as the Basic Law, are under threat, and mass protests two years ago demanded greater democracy.
Pro-independence legislators Baggio Leung and Yau Wai-ching deliberately misread their oaths of office for seats on the Legislative Council (Legco) last month.
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Beijing sees any talk of independence as treasonous and the official Xinhua news agency quoted a spokesman for the government's Hong Kong and Macao Affairs Office welcoming the ruling.
Hong Kong police used pepper spray Sunday to drive back hundreds of protesters angry at China's decision to intervene in the row.
In chaotic scenes reminiscent of the demonstrations of 2014, protesters charged metal fences set up by police outside China's liaison office in the city.
They carried banners reading: "Chinese law interpretation tramples on Hong Kong people."
Today's announcement was the fifth time since the handover that China has interpreted the Basic Law.
"I hope people can see the real face of these people," he added.
Both those promoting independence and those arguing for self-determination were supporting separatism, he said - anathema to China's ruling party.
Ahead of the ruling, the state-run Global Times newspaper, which often takes a nationalistic tone, backed Beijing's intervention.