Hong Kong began free coronavirus testing for all residents on Tuesday, as the Chinese-led initiative faced scepticism from the city's medical community and public.
The initiative began at 8:00 am (0000 GMT), with a 60-strong mainland team conducting tests. It is the first direct help from China's health officials for the semi-autonomous city as it battles the pandemic.
The scheme has emerged as a politically charged issue, with authorities in Hong Kong and China saying critics are trying to smear the central government.
Hong Kong Chief Executive Carrie Lam told a regular press briefing the testing was necessary to find invisible transmissions within society and urged medical workers to treat the testing "objectively and professionally".
The issue comes at a sensitive time with anxiety running high about what many of Hong Kong's 7.5 million residents see as Beijing's efforts to rein in their freedoms, in particular with a national security law imposed in June.
Lam said at the news conference that Hong Kong's executive, legislative and judicial powers are derived from Beijing.
"Our power is from the central government's authorisation to Hong Kong," she said in comments that are likely to stoke further concern that the city has taken a more authoritarian turn.
China's offer to test residents comes as the number of new daily cases has fallen substantially, to single- or low double-digits, from triple-digits during the latest surge in cases a few weeks ago.
Nearly 600,000 people have signed up for the free testing, with people lining up on Tuesday at the 141 test centres across the city.
A Hong Kong pro-democracy union of healthcare workers and several activists called on Sunday for a boycott of the scheme.
Some democracy activists have suggested that people's DNA will be collected and abused under the cover of testing. The government has dismissed that saying no samples would be taken out of the city.
The volume of testing is due to increase to 500,000 a day from about 12,000 previously, authorities have said.