Hong Kong police have made the first arrest under the territory's new national security law. It comes less than 24 hours after the law came into effect and on the same day Hong Kong marks its handover to China from British rule.

Police released a photo on Twitter early Wednesday afternoon. It shows a man bearing a Hong Kong independence flag in Causeway Bay.

Police said they also arrested 10 people for violating the new law and about 360 others for unlawful assembly and other related offences.

It follows a tumultuous start to the day with protestors taking to the streets to voice their opposition to the law.

Beijing approved the new legislation on Tuesday. It lays out four criminal acts it says endangers national security secession, subversion, terrorism, and collusion with foreign or external forces. If convicted, people could face life in prison.

Chief Executive Carrie Lam defended the law during a ceremony to mark the 1997 handover.

Lam said, "The enactment of the law is the most important thing for Hong Kong's relationship to the mainland since the handover. It's a historic step, as it has completed a system to protect our sovereignty and security."

But critics say the law undermines the "one country, two systems" framework which is supposed to guarantee Hong Kong a high degree of autonomy and judicial independence.