Hong Kong media tycoon and pro-democracy supporter Jimmy Lai has been charged with fraud and detained until a court hearing in April next year.
On Thursday a court denied him bail over a charge relating to the illegal use of his company's premises.
It comes a day after three prominent pro-democracy activists were jailed.
The cases have raised fears of a renewed crackdown on the city's activists and media figures, spurred by a controversial new security law.
Mr Lai was arrested under the National Security Law earlier this year and later released on bail.
China has said the new law will return stability to the territory after a year of unrest, but critics say it has silenced dissent.
In another development, former Hong Kong pro-democracy legislator Ted Hui has announced during a trip to Denmark that he is going into exile.
What happened to Jimmy Lai?
Mr Lai, 73, was arrested on Wednesday night along with two other senior executives from media company Next Digital.
Mr Lai is the founder of Next Digital which publishes Apple Daily, a well-read tabloid which is frequently critical of Hong Kong and mainland Chinese leadership.
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On Thursday, the three men appeared in court to face charges relating to the alleged illegal use of their company's headquarters for purposes not permitted by its lease.
The charges stated that they had sublet a section of the premises. The landlord is a corporation established by the government to run the city's industrial parks.
The fraud charge is not being heard under the National Security Law but, according to an Apple Daily report, the judge presiding over the case had been hand-picked by the city's leader, Carrie Lam, to handle national security cases.
A police statement on the arrests did not name those detained but pointed out that one of them - Jimmy Lai - was still under investigation for violating the National Security Law.
While the two senior executives were granted bail, Mr Lai was denied because he was deemed an "absconding risk", according to local reports.
Who is Jimmy Lai?
One of the city's most prominent supporters of the pro-democracy movement, Mr Lai is estimated to be worth more than $1bn (£766m). Having made his initial fortune in the clothing industry, he later ventured into media and founded Next Digital.
In a local media landscape increasingly fearful of Beijing, Mr Lai is a persistent thorn for China - both through his publications and writing that openly criticises the Chinese leadership.
It has seen him become a hero for many residents in Hong Kong. But on the mainland he is viewed as a "traitor" who threatens Chinese national security.
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Earlier this year, he was accused of "colluding with foreign forces" under the National Security Law.
He was arrested in August, becoming the most high-profile person in Hong Kong to be detained under the law, and his newspaper offices were raided by hundreds of police officers.
He was later released on bail.
Interviewed by the BBC before the latest arrest, he said he would not give in to intimidation.
"If they can induce fear in you, that's the cheapest way to control you and the most effective way and they know it.
"The only way to defeat the way of intimidation is to face up to fear and don't let it frighten you."
What did Ted Hui say?
The former Hong Kong pro-democracy legislator said in a statement: "I hereby announce that I will go into exile and will withdraw my membership of the Democratic Party of Hong Kong to leave Hong Kong.
"There is no word to explain my pain and it's hard to hold back tears."
Mr Hui faces charges linked to pro-democracy protests, but had been given bail.
