The free voluntary tests are part of an attempt to stamp out a third wave of infections that began in late June and saw the densely populated city reimpose economically painful social distancing measures.

But the involvement of mainland Chinese testing firms has deterred many in a politically divided city convulsing with resentment towards Beijing's rule.

On Monday morning, civil service chief Patrick Nip said 1.15 million people had signed up since mass testing began last Tuesday out of a city population of some 7.5 million.

That figure is well below the 4-5 million leading health experts said would be needed for a mass testing scheme to be effective at finding and stopping hidden transmission chains.

Nip said the tests would be extended for a further seven days to encourage more people to sign up.

"Please take the opportunity to help Hong Kong end the epidemic's third wave so that people's lives and economic activities can gradually return," he wrote on Facebook.

The tepid enthusiasm is a blow for the city's pro-Beijing leadership, which suffers from low approval ratings.

They had called on residents to embrace the scheme, billing it as a benevolent public health initiative made possible with Chinese help.

But the involvement of teams and labs from the mainland has sent the rumor mills into overdrive and compounded fears of Beijing's surveillance state, which uses biometric data to monitor its citizens.

A group of pro-democracy politicians and lawmakers, as well as a medical union critical of Beijing, called on the public to boycott the test.

Some prominent Hong Kong health experts also questioned the efficacy of a mass testing program, arguing that more targeted monitoring of at-risk and vulnerable communities would be a better use of resources.

They raised concerns that the act of testing so many people might itself help spread the virus in a city where emergency rules currently forbid more than two people from gathering in public.

On Sunday, police arrested nearly 300 people protesting against the government's decision to suspend local elections for a year because of the virus.

Both Beijing and city leader Carrie Lam have accused those opposed to the testing as being politically motivated and "anti-China".

Lam said no DNA or other biometric data would be harvested from the samples, which would not be tested on the mainland.

At the height of the third wave in late July, Hong Kong was recording about 150 new cases a day. 

Over the last two weeks, that number has hovered between 10-20 a day, even with the mass testing scheme under way.

Since the pandemic began, Hong Kong has registered nearly 4,900 infections and 98 deaths from the coronavirus.