Two Hong Kong men have been arrested by national security police for merely possessing what the authorities say are "seditious" children's books - the latest in a slew of arrests over the past week that have caused widespread unease across the city.

Police said two men, aged 38 and 50, were arrested this week in a joint operation with customs officers who raided their homes and offices and found copies of the comic books that allegedly "incited hatred or contempt" against the Chinese and Hong Kong governments and the judiciary. Hong Kong media reported the arrests this week, citing a press release.

Police said the books were "seditious publications that could incite others into using violence and disobeying the law," adding that they were related to a sedition trial that happened last year.

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The books were part of a series of three that portrayed Hongkongers during the 2019 unrest as sheep trying to defend their village from wolves, an apparent reference to Chinese authorities. The titles were ruled seditious in a controversial trial last year in which five speech therapists were sentenced to 19 months in prison for "conspiring to publish, distribute and display three books with seditious intent."

The Chinese-language Mingpao newspaper reported Wednesday that several copies of the books were mailed to the duo from Britain. The men, who have not been named by police, have been released on bail but must report to police next month, according to police officers quoted.

This case is believed to be the first time that police have detained citizens for merely possessing the books. It was not the first time, however, that arrests have been made in connection with the books. In January, a 24-year-old man was arrested by national security police for posting a link to a download site for the sheep and wolves series. He was charged with being involved in an act "with a seditious intention" and was refused bail, according to online news portal Inmediahk.net.

The convictions used a colonial-era sedition law that authorities now often use alongside the national security law to prosecute government critics. The sedition law outlaws incitement to violence, disaffection and other offenses against the administration. Sedition cases are overseen by designated national security judges, and defendants charged with sedition also face a more stringent national security bail assessment.

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The sweeping national security law, imposed by Beijing to stamp out the monthslong, sometimes violent anti-government protests that started in 2019, codifies penalties as severe as life imprisonment for crimes including acts of secession, subversion, terrorism and collusion with foreign forces.

The case has caused widespread unease across the city and many posted comments on social media voicing their fears.

"I don't understand what's so seditious about these comic books? How would we know which books are seditious and which are not?" said one comment on Facebook.

Johannes Chan, former chair of public law at the University of Hong Kong and visiting professor at University College of London, said the arrests highlight the vagueness of the definition of sedition charges. He said people should not be guilty for merely possessing the publications if they didn't know they were seditious or didn't have "seditious intention."

"Otherwise, if a cartoon in [a newspaper] is considered seditious, every single reader who has kept a copy of the newspaper could be guilty of the possession offense. This could hardly be compatible with the guarantee for free speech in the Basic Law or the Bill of Rights," he said.

Hong Kong's Basic Law is a mini-constitution that guarantees that its civil freedoms and rights should remain unchanged for 50 years after Hong Kong's return to Chinese rule, which happened in 1997.

At the time of the speech therapists' 2021 arrests, a senior national security police official said merely possessing those publications "should not pose a problem." But police advised people to destroy the "radical" books because they "instilled in children the ideas to confront and oppose the government."

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This time, the police press release said that "the possession of seditious publications is a serious crime" and warned it could lead to imprisonment.

Despite officials' efforts to promote the city internationally after COVID-19 restrictions were lifted, this was the latest in a series of arrests on national security charges during the past couple of weeks that highlight the fragile state of civil freedoms Hong Kong.

A 23-year-old woman was arrested March 15 under the national security legislation for allegedly publishing messages online advocating Hong Kong independence.

Veteran labor rights activist Elizabeth Tang was arrested a day later on suspicion of "colluding with foreign forces," after visiting her husband - prominent former opposition lawmaker Lee Cheuk-yan - in prison. It's an offense under the national security law.

Two days later, Tang's sister, Marilyn Tang, and a lawyer, Frederick Ho, were arrested for allegedly removing evidence from her home. They were accused of conspiring to pervert the course of justice.