The popular protest slogan "Liberate Hong Kong, revolution of our times" connotes separatism or subversion, the city's government said, pointing to crimes that are covered under the new national security law imposed by Beijing.
The rallying cry appears on placards at rallies, is printed on clothes and accessories and scribbled on post-it notes on walls across the Chinese-ruled city.
It was unclear whether independent courts would uphold the government's view on the slogan, which further stokes fears the new legislation against secession, subversion, terrorism and collusion with foreign forces, crushes free speech in Hong Kong.
"The slogan "Liberate Hong Kong, the revolution of our times" nowadays connotes "Hong Kong independence", or separating the Hong Kong Special Administrative Region (HKSAR) from the People's Republic of China, altering the legal status of the HKSAR, or subverting the state power," the government said in a statement late on Thursday.
The government has repeatedly said the security law won't affect freedom of speech and other rights in the city.
On Wednesday, the 23rd anniversary of the former British colony's handover to Chinese rule, police arrested around 370 people during protests over the legislation, with 10 of those involving violations of the new law.
Critics of the law have slammed the lack of transparency around it ahead of its publication and the speed at which it was pushed through. Beijing unveiled details of the legislation late on Tuesday and the law came too effect on Wednesday.
The new law punishes crimes of secession, subversion, terrorism and collusion with foreign forces with up to life in prison. It will also see mainland security agencies in Hong Kong for the first time and allows extradition to the mainland for trial in courts controlled by the Communist Party.
China's parliament adopted the security law in response to protests last year triggered by fears Beijing was stifling the city's freedoms and threatening its judicial independence, guaranteed by a "one country, two systems" formula agreed when it returned to China.
Beijing denies the accusation.
The law has triggered alarm among democracy activists and rights groups. Demosisto, a pro-democracy group led by Hong Kong activist Joshua Wong, disbanded hours after the legislation was passed, while prominent group member Nathan Law said on Thursday he had left the global financial hub.
"The protests in Hong Kong have been a window for the world to recognise that China is getting more and more authoritarian," Law told Reuters in an interview via internet video.