Wong’s arrest on subversion charges inside prison comes as US Secretary of State Mike Pompeo threatened sanctions over Hong Kong authorities’ mass arrests.
Jailed dissident Joshua Wong has joined the ranks of Hong Kong anti-Beijing figures arrested under a new security law as the United States threatened fresh sanctions in response to Beijing's mounting crackdown in the finance hub.
More than 1,000 police officers detained 53 prominent figures, including a US citizen, in dawn raids on Wednesday on charges of "subversion", a new national security crime that carries up to life in prison.
The sweep was the latest salvo in Beijing's battle to stamp out dissent in the semi-autonomous city after millions hit the streets in 2019 with huge and sometimes violent anti-Beijing protests.
A senior police source told AFP that Wong, who is currently behind bars, was arrested on a new charge of subversion, the first time he has been detained under the new national security law.
The 24-year-old is one of the city's most recognisable opposition figures and is currently serving a jail sentence for his role in organising opposition protests.
Local media reported Tam Tak-chi, another activist currently in custody on a sedition charge, was also arrested for subversion on Thursday morning.
Western nations have accused Beijing of using its crackdown to shred the freedoms that were promised under the "One Country, Two Systems" set-up when the British colony was returned to China.
Secretary of State Mike Pompeo threatened US sanctions over Hong Kong authorities' mass arrests of opposition figures and voiced anger that an American was among those rounded up.
Pompeo said that the 53 people arrested a day earlier "should be released immediately and unconditionally."
"The United States will not stand idly by while the people of Hong Kong suffer under Communist oppression," Pompeo said in a late-night statement after a day of political violence in Washington.
"The United States will consider sanctions and other restrictions on any and all individuals and entities involved in executing this assault on the Hong Kong people," he said.
Pompeo, who remains in office for two weeks, said the United States would also "explore restrictions" against the financial hub's trade representative office in Washington.
Washington has already sanctioned multiple Chinese and Hong Kong officials, including city leader Carrie Lam.
British Foreign Minister Dominic Raab called the detentions "a grievous attack on Hong Kong's rights and freedoms" and said Beijing "deliberately misled the world about the true purpose" of the security law.
Canada, France and the European Union also issued statements condemning sliding freedoms in Hong Kong and called for the release of those detained.
Furor over last year's primary
The alleged offence of those arrested for subversion was to organise an unofficial primary last summer to decide who would stand as candidates for the city's partially-elected legislature in the hopes they might take a majority for the first time.
Many of those candidates were ultimately disqualified from standing and authorities scrapped the election because of the coronavirus.
But Chinese and Hong Kong officials have labelled the primary as an attempt to "overthrow" and "paralyse" the city's government and therefore a threat to national security.
Analysts say it is a new watershed moment for the semi-autonomous city.
"Very clearly and unequivocally it demonstrates the Chinese government's control over Hong Kong," Dylan Loh, an expert on China's politics at Singapore's Nanyang Technological University, said.
"There should be no illusions anymore that the central government is firmly in control," he added.
Those detained are a broad cross-section of Hong Kong's opposition, from veteran former anti-Beijing lawmakers such as James To, Andrew Wan, Lam Cheuk-ting and Claudia Mo to a host of younger activists.
John Clancey, an American national and long term Hong Kong resident who worked at a prominent human rights law firm, was also arrested for subversion.
Clancey was later granted bail, his associate said.
Controversial national security law
Hong Kong police can hold anyone arrested for up to 48 hours before they must be presented in court.
A senior police source told AFP on Thursday that most of those arrested were expected to initially be released on bail later in the day, rather than charged.
Those charged with national security crimes are not usually granted bail.
The national security law was imposed on Hong Kong in late June in response to the 2019 protests, targeting acts Beijing deems to be secession, subversion, terrorism and collusion with foreign forces.
Officials said the security law would target only an "extreme minority" and was needed to restore stability after 2019's huge and often violent anti-Beijing rallies.
The law has swiftly silenced dissent and outlawed a host of peaceful political views, with dozens of prominent figures targeted even before Wednesday's operation.
Over the course of the last year, prominent anti-Beijing supporters have been arrested, jailed, barred from politics or have fled overseas.