HONG KONG — Hong Kong police have made their first arrests under a new national security law imposed by mainland China.

At least seven people were arrested under the law, including two who were carrying a Hong Kong independence flag or a sign calling for Hong Kong’s independence.

The law, which took effect Tuesday night, makes activities deemed subversive or secessionist punishable by up to life in prison. It appears directly aimed at some of the activities that occurred during anti-government demonstrations last year.

The seven arrests under the new law were among more than 180 people detained Wednesday on various charges on the 23rd anniversary of the former British colony’s handover to Chinese rule.

Hong Kong leader Carrie Lam marked the anniversary by strongly endorsing the new law.