HONG KONG — A Hong Kong media tycoon known for his ardent opposition to China was arrested on Friday over his role in a pro-democracy protest last year, the police said, dealing another blow to the city’s independent media.

The tycoon, Jimmy Lai, a rare figure among Hong Kong’s elite for his willingness to take on Beijing, owns Next Media Group, which publishes a popular pro-democracy newspaper and website called Apple Daily. His arrest comes as the city has been dealing with the twin shocks of the protest movement and now the coronavirus outbreak.

His singular status as a prominent businessman in Hong Kong who openly supports the democracy movement and antigovernment protests has made him a frequent target of Beijing-backed elements.

Mr. Lai was arrested on allegations of taking part in an unauthorized assembly on Aug. 31, a police spokesman said Friday. That day, crowds of protesters defied a police ban on their march and clashed with riot officers. Two other veteran pro-democracy activists were also arrested on Friday over their roles in the protest.