Chinese Ambassador to Canada Cong Peiwu branded pro-democracy protesters in Hong Kong as violent criminals and says if Canada grants them asylum that amounts to interference in China's internal affairs.

As Carrie Lam spoke, groups of officers in riot gear conducted stop-and-search operations along an expected protest marching route.

The Hong Kong government has dismissed the concerns, saying that no personal data will be attached to the specimen bottles and that samples will be destroyed in Hong Kong after the exercise.

On July 2, police arrested a 24-year-old man at the airport on suspicion of stabbing an officer during the protest just hours after China's new security law was imposed.

Hong Kong police fired water cannon and tear gas and arrested nearly 200 people on Wednesday as protesters took to the streets in defiance of sweeping security legislation introduced by China that critics say is aimed at snuffing out dissent.

China has long detested Hong Kong's vigil, the only such activity allowed on Chinese territory to commemorate victims of the crackdown, which remains a taboo subject on the mainland.

"We will take action to revoke Hong Kong's preferential treatment as a separate customs and travel territory from the rest of China," Trump said.

The statement came as U.S. President Donald Trump is due to announce later on Friday his response to the Chinese parliament's advancement of national security legislation for Hong Kong.

The city’s pro-democracy opposition sees both the security legislation and the anthem law as assaults on that autonomy, and the U.S. has called on China to back off on the security law.

The move carries significant implications for trade and travel with Hong Kong.