Activists have accused China of carrying out a political purge in Hong Kong after 10 more people were arrested under a new national security law.
Hong Kong’s democracy movement is increasingly gripped by fear, with well-known activist Agnes Chow, 23, becoming the latest target of police.
Friends say the pro-democracy campaigner is under investigation for inciting secession.
Police announced the arrests on Twitter on Monday but did not release the names of the suspects.
However, in an earlier post on her official Facebook page, Ms Chow said police had arrived at her home and her lawyers were on the way. A later post confirmed she had been taken away by police.
Another activist was detained while eight people, including the city’s most high-profile media owner Jimmy Lai, were arrested on Monday.
Mr Lai’s sons, Timothy and Ian, and senior executives at the newspaper’s publishing company were also arrested, according to Mr Lai’s newspaper, Apple Daily.
Videographer Wilson Li, who works for British broadcaster ITV, was also among those arrested, according to a tweet by Hong Kong reporter Ezra Cheung.
The national security law imposed on Hong Kong by Beijing came into force on June 30, the eve of the 23rd anniversary of the handover of the ex-British colony back to China.
It targets secession, acts of terrorism, subversion and collusion with foreign forces and is widely seen as ending the One Country, Two Systems agreement that guaranteed Hong Kong extra freedoms until 2047, part of the conditions for Britain returning the colony to China in 1997.
Sentences for violation of the law range from three to 10 years or, for most serious offences, life imprisonment.
It has garnered widespread international criticism that has resulted in countries including Australia, Britain, the US and Germany suspending extradition treaties with the territory.
Mr Lai is an outspoken China critic, and police said his offences included collusion with a foreign country.
Apple Daily posted footage on social media of dozens of police officers entering the Next Media Building, the newspaper’s headquarters, with images showing police searching news rooms.
Steve Li, the chief superintendent of the national security law, was seen in live footage filmed by Apple Daily staff.
Next Digital, the group founded by Mr Lai, issued a statement accusing police of using “barbaric tactics”.
It accused police of “rifling through documents in the newsroom” of “breaching press freedom through intimidation” and “creating an atmosphere of ‘white terror’ ” – the name used by locals to refer to Beijing’s tightening grip on the city.
Amnesty International issued a statement saying targeting a pro-democracy newspaper threatened press freedom.
Shares Next Digital skyrocketed on the Hong Kong Stock Exchange during Monday afternoon’s trading.
The arrest of Ms Chow, who is from the disbanded group Demosisto, was announced by Joshua Wong, another well-known activist from the disbanded group.
He posted a screenshot Ms Chow had posted on Twitter reading “police had arrived at her house” and “her lawyers were rushing there”.
The US has imposed sanctions on Hong Kong Chief Executive Carrie Lam and 10 other officials for “implementing Beijing’s policies of suppression of freedom and democratic processes”.
China responded on Monday with a list of sanctions against 11 US citizens.