Authorities in Hong Kong told a political party which promotes independence that it might be banned on national security grounds, in one of the most severe steps against separatist voices since the territory returned to Chinese rule in 1997.

The move against the Hong Kong National Party(HKNP) comes at a time authorities have ratcheted up pressure on young democracy activists, some of whom have been jailed, and denounced any pro-independence action as an illegal challenge to Communist Party rulers in Beijing.

Semi-autonomous Hong Kong enjoys freedoms unseen on the mainland, including freedom of expression. Concern is, however, growing those rights are under serious threat from an assertive China under President Xi Jinping.

John Lee, Hong Kong’s secretary for security, said on Tuesday he was considering the request made by police to ban the Hong Kong National Party (HKNP), a well-known but small group with a core membership of around a dozen, which promotes independence from China.

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“In Hong Kong we have freedom of association, but that right is not without restriction,” Lee told reporters.

Questioned how the party was damaging national security, Lee said he could not comment on the details.

However, he added that under Hong Kong law, national security meant safeguarding the “territorial integrity and the independence of the People’s Republic of China”.

In a letter to the leader of the HKNP, 27-year-old Andy Chan, the territory’s security bureau said the group has three weeks to make a case for why it should be allowed to operate.

It is the first time such a ban has been sought since Britain handed sovereignty of Hong Kong back to China in 1997.

‘Pursuit of freedom’

The HKNP is one of a handful groups that openly advocate Hong Kong independence.

Founded in 2016, it drew at least 2,500 people to what was dubbed Hong Kong’s first pro-independence rally two years ago.

“I will never stop in my pursuit of freedom, human rights, equality and dignity,” Chan, who was previously banned from running for a seat in the city’s Legislative Council, told Reuters news agency.

Most people in the city of 7.3 million do not support independence.

Beijing has repeatedly criticised the movement, fearful of the idea taking hold on the mainland.