Hong Kong media tycoon Jimmy Lai became the highest-profile person arrested under a new national security law on Monday.
He was detained over suspected collusion with foreign forces as around 200 police searched his offices.
Lai has been one of the most prominent democracy activists in the Chinese-ruled city.
His company Next Media publishes the local tabloid, Apple Daily.
Meanwhile, China imposed sanctions on 11 U.S. citizens including legislators. They were vocal critics of the sweeping new security law.
Among those targeted were Senators Ted Cruz and Marco Rubio as well as individuals at non-profit and rights groups.
That was in response to the U.S. imposition of sanctions on 11 Hong Kong and Chinese officials accused of curtailing political freedoms in the former British colony.
Relations between the two countries have deteriorated sharply in recent months over issues ranging from trade, to Hong Kong and China's handling of the novel coronavirus.
Beijing imposed the security law at the end of June - and it was condemned by Western countries.
It punishes with up to life in prison anything China considers subversion, secession, terrorism - or collusion with foreign forces.
Critics say it crushes freedoms in Hong Kong, while supporters say it will bring stability after last year's prolonged pro-democracy protests.
On Monday, police said they arrested several people under the law.
According to local media reports, among them, was pro-democracy activist Agnes Chow.