Groups of seemed to be police officers appeared at the airport with their whole body covered, including their Police ID

The evidence of police:

A large group of citizens had gathered at the Hong Kong International Airport since that afternoon. At 10:45pm, the police arrived at the airport to stand-by. At the scene, police officers that wore black face masks, held batons and shields can be seen. Their faces were all covered such that no one can see their appearances. Those police officers did not expose any Police ID.

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The Stand News