The force said officers were conducting patrols in the Ma Tau Wai area on Thursday when they spotted someone writing slogans on several objects such as rubbish bins and scaffolding.
The slogans were said to be linked to the anti-government movement, touching on politically sensitive events such as the July 21 Yuen Long attacks and the August 31 Prince Edward MTR Station violence in 2019.
Police said they found four marker pens and arrested the woman on suspicion of criminal damage.
The force also alleged that the suspect was involved in vandalism at more than 20 other spots in the area.
They accused her of having “ulterior motives” behind her vandalism, saying she had intended to spread false information and damage social harmony by writing the slogans.
They urged people not to challenge the law, noting that those convicted of criminal damage risk up to 10 years behind bars.