On Tuesday, the standing committee of China's legislature voted unanimously in favor of the national security bill, sparking a firestorm of negative comments from the Hong Kong opposition as well as several Western officials, who claim it would undermine the city's autonomy guaranteed during its transition from the United Kingdom to China. By Tuesday evening, Chinese President Xi Jinping signed the bill into law.

"A man was arrested for holding a #HKIndependence flag in #CausewayBay, Hong Kong, violating the #NationalSecurityLaw. This is the first arrest made since the law has come into force," the police tweeted.

The new legislation bans separatist, subversive and terrorist activities, along with any form of foreign interference in Hong Kong.

The move is supported by the Hong Kong leadership but has been met by a wave of protests in the city, as some residents fear their rights may be infringed.

According to Beijing, the new security law aims to punish illegal activities in the city without harming the existing democratic freedoms of locals.

In early June, it was announced that the police would create a special unit to oversee the enforcement of the new legislation.