Choy Yuk-ling’s employer, broadcaster RTHK, said the arrest was linked to her reporting on alleged police misconduct.
Authorities said she had been arrested for illegally using a car registration search as part of an investigation.
Hong Kong journalists have rallied online in support, saying her arrest is a scare tactic to discourage reporting.
RTHK linked Ms Choy’s arrest directly to a documentary she made for the the station about a July 2019 attack on pro-democracy protesters, in which dozens of men armed with sticks, metal rods and bats, and suspected to be triad gangsters, attacked people returning from a demonstration.
The attack, in Yuen Long in north-west Hong Kong, was captured by victims and bystanders on mobile phones and left Hong Kong in shock. Tens of thousands took to the streets in the days afterwards to protest against the violence.
In a tweet confirming Ms Choy’s arrest, RTHK said her documentary “revealed how police were patrolling the town before the rampage, and took no action over the men wielding weapons”.
RTHK chief Leung Ka-wing said the station had been unable to contact Ms Choy after her arrest. Speaking to reporters, he said he was “worried about whether we can continue the way we produce accurate news as before”. Asked if RTHK would put on hold future investigative reporting, he said; “Why should we stop?”
Chris Yeung, the chairman of the Hong Kong Journalists Association, said the authorities were “exhausting every possible way to suppress the press, to make it difficult, if not impossible, for reporters to do their job to dig out the truth”.
A number of politicians also condemned the arrest. Democratic Party lawmaker Lam Cheuk-ting, who was among those injured in the violent attack covered by Ms Choy, described her as a professional journalist who had “asked all the right questions” in the programme.
Civic Party leader Alvin Yeung questioned whether authorities were taking “revenge” on Ms Choy by singling her out for punishment for using tools and techniques used by all investigative journalists.
In August, two Democratic Party politicians were arrested along with 14 others in connection with the violence in Yuen Long.
Lam Cheuk-ting and Ted Hui Chi-fung were held on rioting charges and both later released on bail. In total, dozens of people have been arrested on similar charges in connection with the violence.