HONG KONG—China’s campaign to quash dissent in Hong Kong accelerated with the arrest of pro-democracy media baron Jimmy Lai, sending an ominous sign about the upcoming of a cost-free press and the new restrictions on these hard Beijing’s tightening grip on the former British colony.

Mr. Lai was among the 10 swept up Monday, which include Agnes Chow, a 23-year-old politician who is perfectly acknowledged for serving to to guide pro-democracy protests, in the latest go by authorities to implement a new countrywide stability regulation imposed by the mainland on Hong…