Lam also said their acts caused damages that were "too horrible to see."
Before presiding over a regular Executive Council meeting, Lam told a press conference that rioters' targeted attacks could take the lives of police officers, saying the police will do all they can to trace after those responsible and bring criminals to justice.
The chief executive recounted another violent weekend marred by a terrorism-style, home-made bomb explosion, a slash-neck attack and other violent assaults against police officers in escalating violence committed by masked rioters.
Thugs and villains also wrecked and looted selective stores, beat up residents who disagreed with them, and hurled petrol bombs into a MTR station in operation and police stations, Lam said.
Lam said she had taken note of the damages that were "too horrible to see" by masked rioters who vandalized offices of establishment councilors, set fires to selective stores and attacked police officers.
Such damages and attacks were believed to have political motives targetting councilors of specific political views for the coming district council election due in November, she said.
"I'm afraid such attacks were not random or simple revenge," she said, adding the HKSAR government would try its best to ensure a smooth, fair, open and honest district council election, though overshadowed by such attacks.
Lam said by attacking certain banks, institutions, stores, cafes and restaurants, rioters attempted to silence those people who hold different views and completely disregarded the freedom of speech, safety and rights of other residents.
The chief executive again called on all residents in Hong Kong to condemn violent acts by rioters, saying she respects those participating in peaceful procession but each and every resident should draw a clear line with rioters engaging in extreme violence so as to uphold the rule of law and restore order.