The situation is so bad, "even the introverts are here", as one demonstrator's poster put it.
The app allowed people in China to engage in political discussions with people outside of the country, on topics such as democracy and Taiwan.
The Chinese foreign ministry threatened retaliation after CGTN's licence to broadcast in the UK was suddenly removed by Ofcom on Thursday.
A bid to switch the licence to an entity called CGTNC was also stopped it its tracks, as the media outlet is ultimately controlled by the Chinese Communist Party, Ofcom said.
Thousands of Hong Kongers are moving to Britain since Beijing imposed a strict national security law on the Chinese territory last summer.
Britain says it will open a visa scheme to Hong Kongers from Sunday, but China says it 'interferes in China's internal affairs'
The lockdown in the Yau Tsim Mong district will be in place until every resident is tested - something expected to take around 48 hours.