HONG KONG (Reuters) - Hong Kong on Tuesday kick-started a process which could lead to a ban on a group that promotes secession from China, the first time since the former British colony’s return to Chinese rule in 1997 it has sought to outlaw a political organization.

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The move against the Hong Kong National Party 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.

The Security Bureau wrote a letter to the founder of the party, Andy Chan, telling him he had 21 days to “make representations in writing” as to why it should not be banned, according to the party’s social media page, which posted photos of the letter.

Secretary for Security John Lee said he had not banned the group and that he could only do so after giving it time to submit its response.

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

Hong Kong is governed under a “one country, two systems” principle which promises it a high degree of autonomy and freedoms not enjoyed in China, such as the freedom of speech and freedom of assembly.

But China’s perceived tightening grip over the financial hub has stoked tensions including the “Occupy Central” movement in 2014 that blocked major roads for nearly three months in a failed bid to pressure Beijing to allow full democracy.

Lee wouldn’t give details about what the group had done to trigger a possible ban, though he cited Hong Kong’s Societies Ordinance that states a group could be outlawed “in the interests of national security or public safety, public order or the protection of the rights and freedoms of others”.

“National security” is specifically defined as “the safeguarding of the territorial integrity and the independence of the People’s Republic of China”.

The Hong Kong National Party 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.

Chan told Reuters he would need to consult lawyers on his next step. He pledged to continue pushing for independence.

“I will never stop in my pursuit of freedom, human rights, equality and dignity,” Chan, 27, who was previously banned from running for a seat in the city’s Legislative Council, told Reuters.

Most people in the city of 7.3 million do not support independence. Beijing has repeatedly slammed the movement, fearful of the idea taking hold on the mainland.

President Xi Jinping warned during a visit to the city last year that any attempt to endanger China’s sovereignty would be an act that crosses a “red line”.