HONG KONG (AP) — Hong Kong police on Tuesday arrested six people, including a former organizer of the city’s decades-long annual vigil that commemorated China’s Tiananmen Square crackdown, for allegedly publishing seditious social media posts, in what were the first publicly known arrests under the city’s new national security law.

Secretary for Security Chris Tang said Chow Hang-tung, a former leader of the group behind the vigil, alongside five others, used a social media page to anonymously publish the posts. Police said their acts began in April and that the suspects were targeting a “sensitive date.”

The authorities have not detailed the content of the posts. But the page started publishing a series of posts to mark the upcoming 35th anniversary of the 1989 crackdown, a politically sensitive topic in Hong Kong and mainland China, on April 30.