Hong Kong pro-democracy tycoon Jimmy Lai has been arrested on suspicion of collusion with foreign troops, his aide said.
Mark Simon said the businessman was being held under the controversial national security law China had imposed in June.
Mr Lai supported pro-democracy protests that erupted in Hong Kong last year.
In February, the 71-year-old mogul was charged with illegal assembly and intimidation. He was later admitted to police.
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“Jimmy Lai is being arrested for collusion with foreign powers at the moment,” said Mr Simon, an executive at Mr Lai’s media company Next Digital.
Hong Kong authorities have so far made no public comment on the issue.
Who’s Jimmy Lai?
Mr Lai is estimated to be worth more than $ 1 billion (£ 766m).
The newspaper he founded, Apple Daily, is often critical of Hong Kong and Chinese leadership.
“The institution hates my gut,” Mr. Lai said earlier in an interview with the New York Times. “They think I’m a troublemaker.”
And on June 30, when the security law was passed, the businessman told the BBC that this “speaks the pin of death for Hong Kong”.
He warned that Hong Kong would become as corrupt as mainland China, because “without the rule of law, people who do business here will have no protection”.
In a separate interview with the AFP news agency, Mr Lai said: “I am ready for prison. When it comes, I will have the opportunity to read books I have not read. The only thing I can do is to be positive. “
What is the new security law?
The law makes it easier to punish Protestants, and reduces Hong Kong’s autonomy.
It also gives Beijing powers to shape life in the former British colony in a way it has never had before.
Critics say it effectively restricts protest and freedom of speech – China has said the new law will restore stability on the ground.
The main provisions of the law include that crimes of segregation, subjugation, terrorism and collusion with foreign troops are punishable with a maximum sentence of life imprisonment.