A teenage Hong Kong democracy activist was charged on Thursday with secession, the first public political figure to be prosecuted under a sweeping new national security law Beijing imposed on the city.

Tony Chung, 19, appeared in court two days after he was arrested by plainclothes police in a Hong Kong coffee shop opposite the US consulate, also charged with money laundering and conspiring to publish seditious content.

He was remanded into custody until his next court hearing on January 7 and faces a possible life prison sentence if convicted under the new law.

Chung is a former member of Student Localism, a small group that advocates Hong Kong's independence from China.

The group said it disbanded its Hong Kong network shortly before Beijing blanketed the city in its new security law in late June but kept its international chapters going.

The legislation -- a response to huge and often violent pro-democracy protests that swept the city last year -- outlawed a host of new crimes, including expressing certain political views such as advocating independence or greater autonomy for Hong Kong.

Chung and three other members of Student Localism were first arrested by a newly created national security police unit in July on suspicion of inciting secession via social media posts.

The United States condemned the arrests as "reprehensible" and demanded the release of the young activists.

Tony Chung, 19, is a former member of Student Localism, a small group that advocated Hong Kong's independence from China
Tony Chung, 19, is a former member of Student Localism, a small group that advocated Hong Kong's independence from China AFP / ISAAC LAWRENCE

China and "its Hong Kong proxies crush the promised autonomy of Hong Kong and eviscerate Hong Kong's respect for human rights, including the rights to peaceful assembly and freedom of expression," Secretary of State Mike Pompeo said in a statement.

Amnesty International said the charges showed authorities were wielding the law to criminalise peaceful political expression.

"The intensifying attack on human rights in Hong Kong has been ramped up another notch with this politically motivated arrest in which a peaceful student activist has been charged and detained solely because the authorities disagree with his views," said Joshua Rosenzweig, head of Amnesty's China team.

Speculation has swirled that police moved on Chung because he was hoping to ask for asylum at the US consulate in Hong Kong.

A little-known group calling itself Friends of Hong Kong put out a statement shortly after Chung's arrest on Tuesday saying it had been trying to arrange for Chung to enter the US consulate that day and seek sanctuary.

AFP was not able to independently verify the group's claim and Chung has been unable to comment because he has remained in police custody.

Chung and three others were first arrested by a newly created national security police unit in July on suspicion of inciting secession via social media posts
Chung and three others were first arrested by a newly created national security police unit in July on suspicion of inciting secession via social media posts AFP / ISAAC LAWRENCE

His bail conditions from his first arrest prevented him from leaving Hong Kong.

Asylum claims to the US have to be made on arrival in the country or via a United Nations refugee referral programme.

With some very rare exceptions, consulates and embassies do not tend to grant asylum as doing so could spark a huge diplomatic tussle.

Local media this week reported that four people who may have been trying to help Chung entered the American diplomatic mission on Tuesday but were turned away.

The consulate has declined to comment.

A small but growing number of Hong Kongers have fled the city since Beijing's crackdown on democracy protesters and recent asylum cases are known to have been successful in both Germany and Canada.

China bypassed Hong Kong's legislature to impose the new security law, keeping its contents secret until it was introduced.

It targets a wide array of acts deemed as secession, subversion, terrorism and colluding with foreign forces.

Along with mass arrests and an anti-coronavirus ban on public gatherings it has largely succeeded in stamping out mass protests and dissent.

But the root causes of last year's huge rallies remain unaddressed and the city is still deeply polarised.

Critics say the law's broad wording has delivered a hammer blow to the semi-autonomous city's freedoms.

The legislation also ended the legal firewall between Hong Kong and the authoritarian mainland, empowering China's security agents to operate openly in the city for the first time.

Beijing has said it will have jurisdiction over the most serious national security offences.

Around two dozen people have been arrested under the new law, including newspaper tycoon and staunch Beijing critic Jimmy Lai.

Only two have so far been charged -- Chung and a man who allegedly rode his motorbike into a group of police during a protest.