They defaced the national emblem mounted at the entrance of the office and painted offensive graffiti on the office's exterior.

Lam said the vandalism is in blatant defiance of the nation's sovereignty and an open challenge to the "one country, two systems" principle. It has caused great harm to the SAR, which is governed under the principle, and it hurt the sentiments of people in Hong Kong and on the Chinese mainland.

Lam also strongly denounced the "astonishing and outrageous" attacks in Yuen Long in the New Territories. Late on Sunday night, attackers in white shirts, some wearing masks, assaulted bystanders and commuters at the Yuen Long subway station with bamboo sticks and metal poles. They said they were targeting radical protesters who have disrupted local order.