Thousands of people attended a pro-Beijing rally in Hong Kong Sunday in support of China's decision to effectively bar two pro-independence legislators from taking office, as fears grow of the city's freedoms being under threat.

Beijing's ruling last week preempted a decision by the Hong Kong courts over whether lawmakers Baggio Leung and Yau Wai-ching should be disqualified from parliament after deliberately misreading their oaths of office, inserting expletives and draping themselves with "Hong Kong is not China" flags.

Beijing's interpretation of the city's constitution issued on Monday said that any oath taker who does not follow the prescribed wording of the oath, "or takes the oath in a manner which is not sincere or not solemn", should be disqualified.

On Sunday rowdy crowds, waving Chinese flags, surrounded the government's headquarters in a show of support for Beijing's unprecedented decision, slammed by pro-democracy activists and legal experts as a massive blow to Hong Kong's judicial independence.

Supporters chanted slogans such as "fight against Hong Kong independence, support the interpretation" at the rally, which was attended by pro-Beijing legislators.

"The cancer cells are those who are promoting Hong Kong independence... we will fight them to the end," lawmaker Michael Tien told the crowd who cheered loudly in response.

"China will never, ever tolerate the splitting of the nation," Tien said.

Priscilla Leung, another pro-China legislator who attended the rally, said the lawmakers' behaviour at the swearing-in ceremony "humiliated all of the Chinese people".

Police said that 28,500 people attended the rally.

The Hong Kong High Court's decision into whether Leung and Yau should be disqualified is still pending.

Hong Kong was handed back to China by Britain in 1997 under a "one country, two systems" deal which protects its freedoms for 50 years, but there are growing concerns those liberties are disappearing.