His supporter has said that pro-democracy tycoon Jimmy Lai of Hong Kong has been arrested on charges of collusion with foreign forces.
Mark Simon said the businessman was placed under a controversial national security law imposed by China in June.
Mr Lai supported the anti-democratic protests in Hong Kong last year.
In February, 71-year-old Mogul was charged with unlawful assembly and threatening. He was later granted police bail.
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“Simon Lye is currently under arrest for collusion with foreign powers,” said Mr. Simon, an executive at Mr. Lai’s media firm Next Digital.
Hong Kong authorities have not made any public comment on the issue so far.
Who is Jimmy Lai?
Mr Lai is estimated to be over $ 1bn (£ 766m).
The newspaper he founded is often critical of Apple Daily, Hong Kong and the Chinese leadership.
In an interview with the New York Times, Mr. Lai previously said, “The establishment hates my courage.” “They think I’m a troublemaker.”
And on 30 June, when the security law was passed, the businessman told the BBC that it “rang the death knell for Hong Kong”.
He warned that Hong Kong would become corrupt like mainland China because “without the rule of law, people doing business here would have no protection”.
In a separate interview with AFP news agency, Mr Lai said: “I am ready for prison. If it comes, I will have the opportunity to read books I have not read. The only thing I can do I’m that positive. “
What is the new security law?
The law makes it easier to punish protesters, and reduces Hong Kong’s autonomy.
It empowers Beijing to shape life in the former British colony in a way it never had before.
Critics say it effectively curbs protest and freedom of speech – China has said the new law will return stability to the region.
Major provisions of the law include that the offenses of segregation, sabotage, terrorism and collusion with foreign forces carry a maximum sentence of life in prison.