HONG KONG (Reuters) - Hundreds of Hong Kong pro-democracy protesters on Saturday rallied against the government’s attempt to ban a political party, saying it dealt the most serious threat to the city’s freedom of association since its return to Chinese rule.

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Organizers said 1,200 protested, while the police estimated 600 attended at the peak.

The city’s security chief said on Tuesday the police had advised him to ban the Hong Kong National Party, which openly promotes the city’s secession from China.

The chief, John Lee, has not officially banned the group yet, saying he could not do so before it has a chance to explain itself. He said Hong Kong still has freedom of association, as enshrined in its mini-constitution known as the Basic Law, but “that right is not without restriction.”

But the developments mark the first time since Hong Kong’s handover from British to Chinese rule in 1997 that the government has moved to outlaw a political organization.

“Hong Kong is starting to prosecute people for their speech,” said protester Isabella Yeung, a teacher in her 40s. “The group has not taken any action or caused any violence. All it did was speaking.”

Most of the protesters were not supporters of independence for Hong Kong. But trailing at the end of the march about 50 people, many wearing face masks, waved “Hong Kong independence” flags and shouted: “We are Hong Kongers, not Chinese.”

Piercing through the chants was the shrill of about 100 black whistles - a local metaphor for injustice. Hong Kong has a separate political and legal system from China under a “One Country, Two Systems” principle, which promises the city a high degree of autonomy and a wide range of freedoms, including freedom of speech and freedom of association.

But after a months-long “Umbrella Movement” street occupation in 2014 failed to pressure Beijing to allow full democracy in Hong Kong, calls for independence emerged.

Chinese President Xi Jinping has said any attempt to challenge China’s national sovereignty crosses a red line.

Benny Tai, a law professor at the University of Hong Kong and an initiator of the “Umbrella Movement” said the proposed ban on the Hong Kong National Party was part of a wider trend of using existing laws to restrict political freedoms.

“After the Umbrella Movement, the Communist Party has started to adjust its strategy towards Hong Kong,” said Tai, who joined Saturday’s protests. “Part of it is to use the law to suppress.”