The last Governor of Hong Kong is urging "the Vatican and Catholics everywhere" to protest the arrest of Catholic Cardinal Joseph Zen.
The Vatican has voiced "concern" after Cardinal Zen's arrest on Wednesday.
Zen is one of the most senior religious figures in Asia. He has been vocal in his criticism of Beijing and a controversial National Security Law that came into force in the former British colony two years ago.
He was taken to the Wan Chai police station and charged with "collusion with foreign forces" over his involvement in a humanitarian charity.
The 612 Humanitarian Relief Fund offered financial assistance to pro-democracy protesters to help pay for their legal and medical expenses.
He has since been released on bail.
Lord Patten, the last Governor of Hong Kong and a Patron of the UK-based Hong Kong Watch, said the cardinal's arrest was "yet another outrageous example of how the Chinese Communist Party is hellbent on turning Hong Kong into a police state".
"The arrest of other decent and brave Hong Kong citizens at the same time doubles down on the wickedness of what the Communists are doing," he said.
"Beijing has, perhaps suitably, imposed a policeman, known for his disregard for human rights, as Hong Kong's next Chief Executive. He has brought discredit on the Hong Kong police service, demonstrating that he would not know the rule of law if it hit him in the eye with a plastic baton rod.
"I hope that the Vatican and Catholics everywhere will protest about the arrest of a great Catholic pastor and advocate, and pray for his safety and wellbeing and that of the whole of Hong Kong.
"This will presumably drive a nail into the coffin of the attempts by the Vatican to establish some sort of deal with China's Communists, who regard any sort of religion as a threat to their tyrannous grip on power."
Zen, 90, was the Bishop of the Diocese of Hong Kong from 2002 to 2009.
Responding to news of his arrest, Vatican spokeman Matteo Bruni said, "The Holy See has learned with concern the news of Cardinal Zen's arrest and is following the evolution of the situation with extreme attention."
Also arrested were barrister Margaret Ng, singer Denise Ho and academic Hui Po-keung.
The National Security Law was passed in June 2020 and led to the arrest and imprisonment of a number of pro-democracy activists, including Christian, Joshua Wong.
After the implementation of the law, Cardinal Zen said he was prepared to be arrested and would not stay silent.
"I shall be prudent; I do not seek to offend, but when I deem it necessary, I will say it," he said.
"If such right and proper words are considered to be against their law, I will endure all the suing, trials and arrests. Numerous predecessors have endured similarly. We have seen how God has always helped them."
Commenting on the arrests, Benedict Rogers, Hong Kong Watch's Chief Executive, said they "signal beyond a doubt that Beijing intends to intensify its crackdown on basic rights and freedoms in Hong Kong"
"We urge the international community to shine a light on this brutal crackdown and call for the immediate release of these activists," he said.
Lord Alton of Liverpool, also a Patron of Hong Kong Watch, has raised his concerns about the arrests with the UK's Foreign Secretary, Liz Truss.
He said: "Arresting a 90 year-old Cardinal, along with one of Hong Kong's most internationally respected barristers, a popular singer and a prominent scholar takes the Chinese Communist Party regime and its proxies in Hong Kong to even lower depths of outrageous inhumanity in their repression of dissent in the city.
"The international community must speak swiftly and clearly in condemnation of these arrests, and must accompany rhetoric with action to signal that Beijing will no longer be allowed to act with impunity and get away with such egregious injustice without consequence."