Hong Kong police said Cardinal Joseph Zen, a 90-year-old former bishop of Hong Kong, and three others were all were released on bail with their passports confiscated under its national security law.

"The Hong Kong authorities decision to target leading pro democracy figures, including Cardinal Zen, Margaret Ng, Hui Po-keung and Denise Ho under the National Security Law is unacceptable," British minister James Cleverly said in a statement to parliament.

Zen has long been an advocate of democratic causes in Hong Kong and mainland China, and has spoken out against China's growing authoritarianism under President Xi Jinping.

The autonomy of Hong Kong, a former British colony, was guaranteed under the “one country, two systems” agreement enshrined in a 1984 Sino-British Joint Declaration.

Britain has accused China of breaching that agreement with a security law that punishes offences like subversion with up to life imprisonment has been used to curb dissent and freedoms.

"We continue to make clear to mainland China and to Hong Kong authorities our strong opposition to the National Security Law, which is being used to curtail freedom, punish dissent, and shrink the space for opposition free press and civil society," Cleverly said.