Chu Kai-pong had already served a three-month prison term for sedition in January for wearing and keeping in his luggage clothes and flags with protest slogans.

On Monday, he pleaded guilty to one count of "doing acts with seditious intent", leading to the city's first conviction under the new tougher law.

One of the slogans on the clothing, "Liberate Hong Kong, revolution of our times", had been found to be "capable of inciting secession" in a separate court case.

Chu was arrested for wearing a T-shirt with the offending slogan and a mask bearing protest phrases on June 12 -- a date associated with the huge and sometimes violent democracy protests in 2019.

Chu told police he believed the slogan called for the return of Hong Kong to British rule, the court heard, and he chose the outfit in order to remind the public of the 2019 protests when the phrase was widely used by pro-democracy demonstrators.

Convicting Chu following his guilty plea, chief magistrate Victor So added that two other offences of failing to produce an ID card and loitering were dropped.

Chu, who has been in custody for three months, will be sentenced on Thursday.

Hong Kong enacted a tougher national security law in March, the second legislation of its kind following the one imposed by Beijing in mid-2020 after quashing the protests.

The revised law beefed up the offence of sedition -- a colonial-era offence-- to include inciting hatred of China's communist leadership and upped its maximum jail sentence from two years to seven.

It also punishes five categories of crimes: treason, insurrection, sabotage, espionage and external interference.

Chu's lawyer argued that the maximum he could be given would be two years.

Sedition was created under British colonial rule, which ended in 1997, and was seldom used until Hong Kong authorities revived it in 2020 and charged more than 50 people and four companies.

Critics, including Western nations such as the United States, say the new security law would further erode freedoms and silence dissent in Hong Kong.

But authorities defended the law as necessary to fulfill a "constitutional responsibility", comparing it to a "reliable lock to prevent someone from breaking into (our) home".

As of last month, 301 people have been arrested under the two security laws, with 176 prosecuted and 157 convicted.