Hong Kong pro-democracy tycoon Jimmy Lai was arrested on suspicion of collusion with foreign powers, his aides said.
Mark Simon said the businessman was detained under controversial national security laws imposed by China in June.
Mr. Lai supported the pro-democracy protests that broke out in Hong Kong last year.
In February, the 71-year-old tycoon was charged with illegal gatherings and threats. He was later released on bail by the police.
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“Jimmy Lai is being arrested for colluding with foreign powers at the moment,” said Mr. Simon, chief executive of Lai’s Next Digital media company.
Hong Kong authorities have yet to comment on the matter.
Who is Jimmy Lai?
Mr. Lai is estimated to be worth more than $ 1 billion (£ 766 million).
The newspaper he founded, Apple Daily, regularly criticizes the leaders of Hong Kong and China.
Earlier, Mr. Lai said in an interview with the New York Times: “The establishment hates me. “They think I’m a troublemaker.”
And on June 30, 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 as corrupt as mainland China because “without the rule of law, the people doing business here would have no protection”.
In a private interview with the AFP news agency, Lai said: “I am prepared for imprisonment. If it comes, I will have a chance to read books that I have not yet read. The only thing I can do. is to be active. “
What is the new security law?
This law makes it easier to punish protesters, and reduces Hong Kong’s autonomy.
It also gives Beijing the power to shape life in former British colonies in ways it never had before.
Critics say it effectively blocks free protests and free speech – China says the new law will return stability to the territory.
The main provisions of the law include crimes of secession, subversion, terrorism and collusion with foreign powers, which can result in life imprisonment.