Hong Kong cops have made their first arrest under a controversial new national-security law imposed by Beijing, arresting a protester Wednesday for carrying a flag calling for Hong Kong’s independence. The man was arrested after police issued multiple warnings to a crowd that they might be in violation of the new law, according to a police statement on Twitter. Hong Kong police said on Facebook they had arrested over 30 people on various charges, from unlawful assembly to the violation of the national-security law. The new law was imposed by China after last year’s anti-government protests. It makes secessionist, subversive, or terrorist activities illegal, as well as foreign intervention in the city’s internal affairs. The law allows for a maximum punishment of life imprisonment. Wednesday marked the 23rd anniversary of the territory’s handover from Britain, and the law was passed ahead of the day’s traditional pro-democracy rallies.