Hong Kong declared illegal a key slogan chanted by hundreds of thousands of pro-democracy protesters over months of rallies, the latest sign that authorities plan to use a new Beijing-drafted national security law to enforce limits on free speech.

In what it called a "solemn statement” late Thursday, the Hong Kong government said the rallying cry "Liberate Hong Kong! Revolution of our time!” was now illegal under the legislation barring secession, terrorism, subversion of state power and collusion with foreign forces. The sweeping law imposed by China, which carries prison sentences as long as life in prison, was made public only as it took effect late Tuesday.

The slogan was just one of several, including also "Hong Kongers, build a nation,” deemed as a threat to national security in guidelines issued to police, according to a person who has seen the document. The rules also barred the waving of flags that advocate independence of Tibet, Taiwan, Shanghai and East Turkestan as similar offenses under the law.