A teenage Hong Kong 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 charged with secession, money laundering and conspiring to publish seditious content, two days after he was arrested in a Hong Kong coffee shop opposite the US consulate.

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

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

The legislation outlawed a host of new crimes, including expressing 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.

However, Chung was arrested again on Tuesday morning by plainclothes police just metres away from the US consulate.

A little-known group calling itself Friends of Hong Kong put out a statement shortly afterwards saying it had been trying to arrange for Chung to enter the US consulate that day and apply for asylum.

Chung was held by police until his appearance in court on Thursday morning. He was denied bail.