A tweet from the 23-year-old pro-democracy figure's account said he was arrested when reporting to Central Police Station on Thursday afternoon.
Hong Kong pro-democracy activist Joshua Wong has been arrested for "unlawful assembly" related to a 2019 protest against a face-mask ban.
A post from the 23-year-old pro-democracy figure's Twitter account on Thursday said he was "arrested when reporting to Central Police Station at about 1pm".
“The arrest is related to participating in an unauthorised assembly on October 5 last year," the post said. Mr Wong said he was told he had “violated the draconian anti-mask law as well”.
About three hours later, Mr Wong returned to Twitter to say he had been released. "Nothing to celebrate on this bizarrely prompt release," he said, adding that he was facing the possibility of a five year prison sentence.
Mr Wong's lawyer Jonathan Man said the activist was arrested when he reported to a police station concerning another case against him, for which he is currently on trial.
"Wong is accused of participating in an unlawful assembly on 5 October last year, when hundreds marched to oppose an anti-mask ban the government rolled out," he said.
The march that day came after much of the city had ground to a halt with the subway suspended and many shops and malls shuttered following a night of violence.
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 masks by invoking colonial-era emergency powers not used for half a century.
The act of resistance came after a night of widespread chaos as hardcore protesters trashed dozens of subway stations, vandalised shops with mainland China ties, built fires and blocked roads. Many chanted "No rioters, only tyranny" and other popular protest slogans.
At the time of the march, Hong Kong had already been battered by four months of increasingly violent pro-democracy protests.
The Hong Kong government invoked a colonial-era emergency law last year on 4 October to ban the wearing of masks at demonstrations, sparking an outcry and mass protests against the ban across the city the next day.
Veteran activist Koo Sze-yiu of the League of Social Democrats has also been arrested on suspicion of participating in the same demonstrations as Mr Wong, according to the Hong Kong Free Press.
Mr Wong, the most high-profile activist in Hong Kong's pro-democracy movement, was a key part of the territory's ongoing protests against China's rising influence.
He has previously spoken out about his fear of being arrested by Hong Kong authorities.
Speaking in a virtual appearance at the Edinburgh International Book Festival in August, he said he was tailed by an unknown car with a China-Hong Kong licence.
While visiting Victoria Peak in Hong Kong with friends, he said he was photographed and verbally abused by a "pro-Beijing gang".
"Those are the common tactics that they use to target pro-democracy activists," he said.
Mr Wong said the price he paid in being under surveillance was "comparatively small" compared to protesters shot with live rounds during demonstrations last year.
But he said it was hard to plan for the future under the threat of the national security law China imposed on Hong Kong in June in response to last year's huge and often violent pro-democracy protests.