A man was detained for carrying a flag that called for independence, while a woman was also held for carrying a sign bearing the same message with a British flag.

Police had made multiple warnings to a crowd in the Causeway Bay shopping district - telling them that they might be in violation of the law.

Overall, at least 180 people have been arrested on various charges - including unlawful assembly, possession of weapons and violating the national security law.

The new law makes secessionist, subversive, or terrorist activities illegal, as well as foreign intervention in the city's internal affairs.

Any person taking part in pro-independence activities, such as shouting slogans or holding up banners and flags, is in violation of the law - irrespective of whether violence is used.

Critics say it effectively ends the "one country, two systems" framework under which Hong Kong was promised a high degree of autonomy.