HONG KONG—China’s coast guard detained a group of Hong Kong activists after intercepting a boat in the South China Sea en route to Taiwan, where the passengers planned to seek refuge from Beijing’s national-security crackdown.
The group, people familiar with the matter said, included Andy Li, an activist who had been arrested in Hong Kong two weeks ago during a swoop by a new national-security police unit that also raided the office of a newspaper. Mr. Li had been released on bail by police, who said he was being investigated…