Police in Hong Kong raided a museum that commemorated the 1989 Tiananmen Square massacre.
Later, officers were seen carrying exhibits from the June 4 Museum.
On Wednesday, four members of the group, the Hong Kong Alliance that managed the museum were arrested. They included Chow Hang Tung, a prominent pro-democracy activist, and lawyer.
The national security law that has a maximum sentence for life imprisonment allows for the arrests.
According to her lawyers, Ms. Chow was accused by police of inciting subversion.
The group is well-known for organizing the annual Tiananmen Square vigil. However, it denies being an agent of foreign agents.
Officials closed the museum in June. The newly formed national security unit carried out Thursday’s raid.
One of the exhibits that was removed was a paper model of Goddess of Democracy, a symbol of 1989 pro-democracy student movement.
Officers were also seen taking photos of large candlelit vigils to Tiananmen victims.
The national security unit had previously requested information from the Hong Kong Alliance, including financial records and personal details of all members.
The deadline was Tuesday and the members of the alliance handed in a letter explaining why they refused to cooperate.
Police officers arrested members of the alliance’s standing committee at their offices or homes the next morning.
Ms. Chow is a human rights lawyer and was charged with encouraging unauthorised assembly. Later, she was released on bail.
On Wednesday, she was due to represent an opposition politician at a bail hearing under the national security laws. However, she was taken into custody at her office just before she could appear in court.
China recently passed a broad-based national security law in Hong Kong, which criminalizes secession, terrorist acts, and collusion with foreign troops.
Critics claim it is designed to crush dissent, but China insists it is intended to maintain stability.