Prominent Hong Kong activist Joshua Wong was arrested Thursday for taking part in a protest at the height of the city's pro-democracy unrest last year, but he vowed to continue resisting China's crackdown on dissent.

The arrest of the territory's most high-profile dissident is the latest in a string of arrests of government critics and comes after China imposed a sweeping new national security law on Hong Kong in late June.

Wong was arrested for "unlawful assembly" over a 2019 demonstration against a government ban on face masks that was imposed before the coronavirus pandemic, his lawyer said.

Later Thursday the European Union criticised the move, saying it was "the latest in a troubling series of arrests of pro-democracy activists since the summer".

Wong, 23, who now faces three separate court cases, said after being bailed that he was also held for violating the "draconian" anti-mask law, which has since been ruled unconstitutional.

Pro-democracy activist Joshua Wong speaks to the media outside the Legislative Council in Hong Kong after learning he had been barred from standing in upcoming local elections
Pro-democracy activist Joshua Wong speaks to the media outside the Legislative Council in Hong Kong after learning he had been barred from standing in upcoming local elections AFP / Anthony WALLACE

Wong's lawyer told AFP he was re-arrested when he reported to a police station concerning another case currently being tried.

"Wong is accused of participating in an unlawful assembly on October 5 last year, when hundreds marched to oppose an anti-mask ban the government rolled out," lawyer Jonathan Man said.

A police spokesman confirmed a 23-year-old was arrested for "knowingly participating in unauthorised assembly" while violating the mask ban.

Wong told reporters after he was bailed: "No matter what happens, I will continue to resist and hope to let the world to know that how Hong Kongers choose not to surrender."

At the time of the October 5 march, Hong Kong had already been battered by four months of increasingly violent pro-democracy protests.

Pro-democracy activist Joshua Wong leaves Central police station in Hong Kong
Pro-democracy activist Joshua Wong leaves Central police station in Hong Kong AFP / ISAAC LAWRENCE

The city had ground to a halt following a night of chaos in which hardcore protesters trashed dozens of subway stations, vandalised shops with mainland China ties, built fires and blocked roads.

Hundreds of protesters, almost all masked, staged the unsanctioned demonstration through the popular shopping district of Causeway Bay, a day after the city's leader Carrie Lam outlawed face coverings by invoking colonial-era emergency powers not used for half a century.

Under Hong Kong's current anti-virus measures, face masks are now mandatory in all public places.

China's security law, which was imposed in late June, was designed to stamp out the demonstrations and targets acts deemed to be secession, subversion, terrorism and foreign collusion.

Beijing has described it as a "sword" hanging over the heads of its opponents as it pushes to return stability. Critics say it has blanketed the city in fear, and UN rights experts warned its broad wording posed a serious risk to Hong Kong's freedoms.

Wong -- who spent most of his teenage years leading protests and has twice been jailed -- recently told AFP he constantly wonders how long it will be before the police's new national security unit comes for him.

The security law has already swept up two of his closest comrades.

Fellow former student leader Nathan Law has fled to Britain and is now wanted for national security crimes, according to Chinese state media.

Agnes Chow -- who has led protests alongside Wong since they were just 15 -- is one of 22 people arrested under the new law so far. She has been released on bail.

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