Police on Wednesday announced their first arrest under Hong Kong's new national security law.

On social media, the force said they had arrested a man in Causeway Bay for allegedly possessing a Hong Kong independence flag.

"A man was arrested for holding a HK Independence flag in Causeway Bay, Hong Kong, violating the National Security Law. This is the first arrest made since the law has come into force," the police said on Twitter.

The post included photographs of an independence flag. Standing next to it was a man wearing a T-shirt that said "Free Hong Kong".

On Facebook, the force said police had intercepted the "suspicious looking man" at about 1.30pm in Paterson Street.

It wasn't clear which offence in particular the man was suspected of committing.

Earlier on Wednesday, Basic Law Committee member Albert Chen said waving a Hong Kong independence flag would be a "grey area" under the new law, and would only be an offence if the bearer was involved in organising, planning, committing or participating in acts that threaten national security.