A Hong Kong man on Monday pleaded guilty to sedition for wearing a T-shirt with a protest slogan, becoming the first person convicted under the city's new national security law passed in March.

Chu Kai-pong, 27, pleaded guilty to one count of "doing with a seditious intention an act".

Under the new security law, the maximum sentence for the offence has been expanded from two years to seven years in prison and could even go up to 10 years if "collusion with foreign forces" was found involved.

Chu was arrested on June 12 at a MTR station wearing a T-shirt with the slogan “Liberate Hong Kong, revolution of our times” and a yellow mask printed with “FDNOL”- the shorthand of another slogan "five demands, not one less”.

Both slogans were frequently chanted in the huge, sometimes violent pro-democracy protests in 2019 and June 12 was a key kick-off day of the months-long unrests.

Chu told police that he wore the T-shirt to remind people of the protests, the court heard.

Chief Magistrate Victor So, handpicked by the city leader John Lee to hear national security cases, adjourned the case to Thursday for sentencing.

Hong Kong was returned from Britain to China in 1997 under Beijing's promise of guaranteeing its freedoms, including freedom of speech, would be protected under a "one country, two systems" formula.

Beijing imposed a national security law in 2020 punishing secession, subversion, terrorism or collusion with foreign forces with up to life in prison, after the months-long protests in the financial hub.

In March 2024, Hong Kong passed a second new security law -- a home-grown ordinance also known as "Article 23" according to its parent provision in the city's mini constitution, the Basic Law.

Critics, including the U.S. government have expressed concerns over the new security law and said the vaguely defined provisions regarding "sedition" could be used to curb dissent.

Hong Kong and Chinese officials have said it was necessary to plug "loopholes" in the national security regime.