Ms Chow is vice chairwoman of the Hong Kong Alliance which organises annual vigils for victims of Beijing’s deadly crackdown on democracy protesters.
She has been arrested for promoting unauthorised assembly.
It comes as Hong Kong has banned the vigil for the second year running, citing coronavirus restrictions.
Police have closed off Victoria Park, where citizens usually gather each year to mark the anniversary. Thousands of officers have been placed on standby to stop any attempt to hold the event.
Hong Kong and Macau are the only places in Chinese territory where people can commemorate the deadly 1989 crackdown on pro-democracy protesters.
But this year, authorities in Macau also banned the vigil for the second year in a row, saying it would violate local criminal laws.
However, Ms Chow continued to call on residents to commemorate the anniversary in their own ways.
“Turn on the lights wherever you are – whether on your phone, candles or electronic candles,” she had posted on Facebook a day before her arrest.
Ms Chow was arrested early in the morning outside her office by officers in plain clothes, according to reports.
She was placed in a black saloon car and driven away, the AFP news agency said.
Speaking to the BBC ahead of her arrest, Ms Chow, who is also a lawyer and a human rights activist, said she was prepared for the inevitable.
“I am prepared to be arrested. This is how Hong Kong is now. If you fight for democracy under an authoritarian regime, being arrested is unavoidable. Let it come. I am willing to pay the price for fighting for democracy,” she said.