Hong Kong democracy campaigner Joshua Wong has pleaded guilty to a charge that could see him sentenced to life in prison.

Seen in court wearing a dark blue shirt and his trademark black-rimmed glasses, the prominent activist, 29, admitted collusion with foreign entities – a crime under the territory's national security law.

According to a charge sheet seen by Reuters news agency, he was accused - along with fellow activist Nathan Law - of conspiring to ask foreign countries, institutions, organisations or individuals outside China to impose sanctions or blockades.

It carries a maximum sentence of life in prison.

Wong is already in jail for another national security offence of conspiracy to commit subversion, relating to an unofficial primary election he and other activists held in 2020.

He has dedicated his entire adult life to fighting for democracy in Hong Kong, which has been the subject of an increasingly tight crackdown on civil liberties by China.

Any form of protest has now been effectively outlawed by the national security law imposed by Beijing in 2020, which followed large-scale demonstrations the year before.

Wong was a leader of the so-called Umbrella Movement and was only 17 when he was first arrested in 2014. He has since been arrested nine more times.

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Wednesday's guilty plea came less than two weeks after two other prominent Hong Kong democracy activists were convicted under the same law.

Lee Cheuk-yan and Chow Hang-tung were organisers of the city state's decades-old vigil commemorating the 1989 Tiananmen Square crackdown, which was banned in 2020.

The law has also been used to jail pro-democracy former media tycoon Jimmy Lai.

Critics say Xi Jinping's government has eroded civil liberties it had promised to preserve for 50 years when Britain handed over the city state in 1997. After Hong Kong's crackdown, some residents have been able to come to the UK via a special visa since 2021.

Among those who moved to Britain was Nathan Law, who Hong Kong authorities are seeking and have offered a reward of HK$1m ($94,400) for information leading to his arrest.

Mr Law and Wong were co-founders of a political party, Demosisto, in 2016, before Hong Kong imposed its national security law, and after Law left, prosecutors say Wong supported the ongoing pro-democracy campaign from the territory by sharing Law's social media posts, in a bid to draw the attention of foreign countries and companies.