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.