Ninety-year-old retired Catholic cardinal Joseph Zen hosted a packed mass dedicated to Chinese churches on Tuesday, hours after appearing in a Hong Kong court over charges linked to the city's pro-democracy protests.

Zen, one of Asia's highest-ranking Catholic clerics, was among five prominent democracy advocates -- including activist and singer Denise Ho and veteran human rights barrister Margaret Ng -- who were detained earlier this month under the city's national security law.

The group acted as trustees of a now-defunct fund that helped pay legal and medical costs for those arrested during huge and sometimes violent democracy protests three years ago.

They were arrested for "conspiracy to collude with foreign forces" but have not yet been charged with that offence, which can carry a life sentence under the sweeping security law imposed by Beijing in 2020.

Instead, all five of the fund's former trustees and its secretary were charged Tuesday with failing to register it as a "society" with police -- a non-national security offence that can incur a fine of up to HK$10,000 (US$1,274) for a first conviction. All pleaded not guilty.

By the evening, Zen was preaching to hundreds of fellow Catholics who had queued to watch him hold mass at the Holy Cross Church on the east of Hong Kong Island.

Ninety-year-old Zen (2nd left) is one of Asia's highest-ranking Catholic clerics
Ninety-year-old Zen (2nd left) is one of Asia's highest-ranking Catholic clerics AFP / Peter PARKS

Many who came to see him were dismissive of the charge against him.

"We of course must respect the rule of law but wielding law as a weapon to oppress is by no means the goal of rule of law," Louise, an educator, told AFP.

Zen dedicated the mass to churches in China, where millions cannot freely practise their belief under the officially atheist Communist Party, which exercises strict control over all recognised religious institutions.

In his sermon, Zen again criticised a deal between China and the Vatican which allows Beijing to nominate bishops for the pope's approval, calling it "unwise" despite being made with "good intention".

"There is an urge to unify those above the ground and those underground but it seems that time is not ripe yet," he said, an apparent reference to underground clergy in China who refuse to comply with state demands.

He also dedicated prayers to churches in Myanmar led by Cardinal Bo, his most outspoken supporter in Asia, as well as "brothers and sisters who cannot join tonight because they are not free".

Zen was among five prominent democracy advocates -- including human rights barrister Margaret Ng -- detained earlier this month
Zen was among five prominent democracy advocates -- including human rights barrister Margaret Ng -- detained earlier this month AFP / Peter PARKS

Zen and the other activists' trial over the fund registration will begin on September 19.

Diplomats from multiple European countries including Germany, France, Sweden and Italy attended the hearing on Tuesday.

Zen's arrest in particular has triggered outrage from Western nations, who have accused China of eviscerating the freedoms it once promised Hong Kong.

But on Monday, the city's security minister told local media the criticism was a "classic smearing campaign".

"To my understanding, the Vatican is a place to pursue justice and peace. If we did not act in accordance with the law because of one's role in the Holy See, then I think it would actually breach the Vatican's principle of justice," said Chris Tang in an interview with the South China Morning Post.

In a separate case on Tuesday, another high-profile democracy campaigner, Benny Tai, was jailed for 10 months over a 2016 media campaign around the city's legislative elections.

Tai, a legal scholar, had earlier pleaded guilty to breaching Hong Kong laws limiting who can place election advertisements.

Despite not being a candidate himself, Tai had promoted a way of coordinating voters to win the most seats for the city's democrats, District Judge Anthony Kwok said.

"(Tai's) actions could have damaged the election's fairness and disadvantaged pro-establishment and other candidates," Kwok said.

Tai is already in custody as he is among a group of 47 democrats arrested for subversion under the national security law.

Rights group Amnesty International blasted Tuesday's sentence, calling it "another politically motivated attack based on his pro-democracy activism".