One of the most senior Catholic clerics in Asia, Cardinal Joseph Zen and three others who helped run a now-disbanded Hong Kong fund for protesters who were arrested on charges of "collusion with foreign forces," were later released on bail.
The 90-year-old former bishop of Hong Kong, was questioned for several hours on May 11 at the Chai Wan Police Station close to his church residence, Reuters reported.
Accused of colluding with foreign forces, they were later released on police bail.
The silver-haired Zen, wearing a white clerical collar, left without commenting to the media.
While he was being detained Vatican News reported, "The Catholic Church in Hong Kong says it is 'extremely concerned about the condition and safety' of Cardinal Joseph Zen Ze-kiun, and offers 'special prayers for him.'"
"I have spoken with Cardinal Zen," Hong Kong Bishop Stephen Chow Sau-yan wrote on his Facebook page. "He told me to let his friends know that he is fine. Not to worry. And he wants us to take a low-profile approach for him," Catholic News Service
Local police said in a statement that the national security department of the police force had arrested two men and two women, ranging from 45 to 90 years old for "collusion with foreign forces" on Tuesday and Wednesday.
The other detained with Zen were Cantopop singer and actor Denise Ho, ex-legislator Margaret Ng, and academic Dr Hui Po Keung, the BBC reported.
If found guilty, they could face life in prison. Human Rights Watch called it a "shocking new low for Hong Kong."
Hong Kong Police told the BBC that the group was suspected of appealing to foreign countries or organisations to impose sanctions on Hong Kong, hence threatening China's national security.
Cardinal Zen fled Shanghai for Hong Kong after the communists took over China 70 years ago. He has long been a critic of the government in Beijing, speaking out for Catholics in mainland China and for more democracy in Hong Kong.