Hong Kong police on Wednesday arrested four leaders of a group that organizes the Asian financial hub's annual Tiananmen Square massacre vigil.

Why it matters: The arrest of barrister Chow Hang Tung and other Hong Kong Alliance in Support of Patriotic Democratic Movements of China activists marks the latest setback for the city's pro-democracy movement. The group has been accused of foreign collusion, in violation of the repressive national security law.

• It comes amid a wider crackdown on groups that the government deems guilty of national security offenses or "unpatriotic" conduct.

Driving the news: In a Facebook post, the activist group shared details of a letter police sent them on Tuesday.

• The police accused the alliance of being "an agent of foreign forces" and demanded it hand over details about members by the end of Tuesday, or risk facing six months in prison or a HK$100,000 (US$13,000) fine.
• The group said it would not comply.

The big picture: Hong Kong police banned this year's June 4 vigil marking the anniversary of the 1989 massacre by Chinese government forces in Beijing's Tiananmen Square.

• Chow, vice chair of the alliance, was arrested the same day for promoting an unauthorized assembly.

Go deeper: Beijing's national security law takes a toll in Hong Kong