Police Commissioner Raymond Siu says mass rallies organised by the Civil Human Rights Front (CHRF) in recent years are suspected of violating the national security law.
The group, which is reportedly on the verge of disbanding, has not held any protests since the legislation was implemented on June 30 last year. The authorities say the law is not retroactive.
But the Ta Kung Pao newspaper quotes Siu as saying the police will investigate whether the CHRF has breached the security law.
"In recent years, [the CHRF] has organised a series of mass rallies, some of them are suspected of violating the national security law. The force will investigate thoroughly," he reportedly told the newspaper.
The front had organised Hong Kong's biggest protest marches since the handover, including those against the extradition bill in 2019 and proposed Article 23 national security legislation in 2003.
Speaking with the Ta Kung Pao, Siu said the CHRF had never registered with the authorities since its establishment in 2002.
He said police are always collecting evidence and can take action against unlawful groups at any time.
Siu added that even though well-known figures from the group are already behind bars, police could still investigate other key members and charge them.
Back in April, the force accused the CHRF of breaching the Societies Ordinance and demanded information from the group on its finances and activities.
Since the introduction of the security law, dozens of pro-democracy groups have disbanded or ceased operations.
Many members of the CHRF had already quit the group over earlier reports that it was under investigation.