Hong Kong authorities on Wednesday banned school students from singing of "Glory to Hong Kong", the unofficial anthem of the pro-democracy protest movement, just hours after Beijing set up its new national security bureau in the Chinese-ruled city.
New security legislation imposed on Hong Kong by Beijing requires the Asian financial hub to "promote national security education in schools and universities and through social organisations, the media, the internet".
The school anthem ban will further stoked concerns that new security laws will crush freedoms in China's freest city, days after public libraries removed books by some prominent pro-democracy figures from their shelves.
Authorities also banned protest slogans as the new laws came into force last week.
The sweeping legislation that Beijing imposed on the former British colony punishes what China defines as secession, subversion, terrorism and collusion with foreign forces, with up to life in prison.
Secretary for Education Kevin Yeung, responding to a question from a lawmaker, said students should not participate in class boycotts, chant slogans, form human chains or sing songs that contain political messages.
"The song 'Glory to Hong Kong', originated from the social incidents since June last year, contains strong political messages and is closely related to the social and political incidents, violence and illegal incidents that have lasted for months," Yeung said. "Schools must not allow students to play, sing or broadcast it in schools."
Earlier on Wednesday, China opened its new national security office, turning a hotel near a city-centre park that has been one of the most popular venues for pro-democracy protests into its new headquarters.
Both Hong Kong and Chinese government officials have said the new law is vital to plug gaping holes in national security defences exposed by the anti-government and anti-China protests that rocked the city in the past year.
They have argued the city failed to pass such laws by itself as required under its mini-constitution, known as the Basic Law.
Critics of the law see it as a tool to crush dissent, while supporters say it will bring stability to the city.
In a statement last month, China's Hong Kong Liaison Office, Beijing's top representative office in the city, blamed political groups "with ulterior motives" for "shocking chaos in Hong Kong education.