Peter Wai, 38, and Bill Yuen, 65, both dual British and Chinese nationals, face charges of assisting a foreign intelligence service and foreign interference.

The case is believed to be the first prosecution under the 2023 National Security Act involving China, according to police.

"The defendants engaged in shadow policing operations on behalf of the Hong Kong Special Administrative Region, and thereby the People's Republic of China," prosecutor Duncan Atkinson told the jury.

They gathered information about "persons of interest" to Hong Kong and undertook surveillance, as if they were entitled to "when no such entitlement existed", he added.

They charges, which also including forcing entry to a address, took place between December 2023 and May 2024.

Yuen had been working at the Hong Kong Economic Trade Office (HKETO) in London, the court was told.

Wai was a special constable with the City of London Police.

But prosecutors said Yuen's role at HKETO went beyond administrative work to include gathering intelligence "useful" to Hong Kong authorities.

- Hong Kong diaspora -

Yuen tasked several individuals including Wai with "various activities" to benefit Hong Kong and China's interest, the prosecutor said.

The two men undertook information gathering, surveillance and acts of deception, primarily targeting members of the Hong Kong diaspora in the UK, he added.

Tens of thousands of people, including democracy activists wanted by Chinese authorities, have moved to the UK since Hong Kong enacted a draconian National Security Law in mid-2020.

Yuen and Wai's alleged activities coincided with Hong Kong authorities publishing bounties of pound100,000 ($133,000) for information helping to identify several UK-based activists, including Nathan Law, jurors were told.

Messages on Yuen's phone showed surveillance of Law began as early as 2021, said the prosecution.

Wai also faces a charge of misconduct in a public office, for allegedly abusing his access to the UK Home Office computer system as a serving Border Force officer.

The case is expected to last up to nine weeks. Both men deny all the charges.