Hong Kong media mogul and one of the city's most prominent pro-democracy figures, Jimmy Lai, was arrested on Monday under Beijing's new national security law, according to a close aide and a police source.
A police source told AFP, the 71-year-old was arrested on charges of colluding with foreign forces and with fraud.
He was already facing other charges relating to the organisation and promotion of pro-democracy protests as well as attending a Tiananmen Square massacre vigil in early June.
"Jimmy Lai is being arrested for collusion with foreign powers at this time," Mark Simon, a colleague of Mr Lai, wrote on Twitter.
The new law brought in by Beijing on 30 June, punishes anything China considers as subversion, secession, terrorism and collusion with foreign forces with up to a life in prison.
Mr Lai has previously told the BBC that the law "spells the death knell for Hong Kong".
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Worth an estimated $1billion (AUD $1.3 billion), Mr Lai has long been a staunch critic of Beijing.
"The establishment hates my guts," Mr Lai he told the New York Times in August 2019.
"They think I'm a troublemaker."
The newspaper he founded, Apple Daily, is frequently critical of Hong Kong and Chinese leadership.