While details of the allegations and legal arguments will only become clear as the proceedings unfold, Tam’s case raises questions over whether the colonial-era laws are still relevant and, if so, why there was such an urgent need for Beijing to enact its own legislation in June.

What do we know about the colonial-era seditious offences?

Against a backdrop of rising hostilities from pro-China forces in the early 20th century, the colonial British government decided to introduce a list of offences – enumerated in the Sedition Ordinance of 1938 – outlawing “hatred or contempt or disaffection” towards the persons of the monarch and the colonial administration.

The law later became part of the Crimes Ordinance during reforms in 1971. Currently, offenders convicted of sedition can be punished by a fine of up to HK$5,000 and two years in prison for their first offence.

Typically deployed by the colonial government to target pro-China factions, the legislation has long attracted criticism.

Not only do those uttering seditious words face penalties, but those who commit an act with seditious intention or publish anything seditious can also be prosecuted, as can anyone in possession of seditious publications, though they face a lighter fine and shorter jail term.

But since the law’s introduction, the city has put in place the 1991 Hong Kong Bill of Rights Ordinance, which stipulates that local laws must comply with the principles of the UN’s International Covenant on Civil and Political Rights. Under the covenant, limitations to free expression must be necessary, reasonable and well defined.

But accepting the “political reality” that Hong Kong would be duty-bound to enact a national security law after the handover, the legislature made a concession instead by narrowing the scope of the law and stipulating sedition must be accompanied by intention to cause violence or public disorder. However, the scaled-back version was never put in place by the Hong Kong government, according to Fu.

What are the differences between the colonial-era sedition offence and the national security law imposed by Beijing?

For a start, the colonial version targets seditious acts, while the national security law enacted by Beijing specifically categorises offences into broadly defined acts of secession, subversion, terrorism and collusion with foreign forces to endanger national security. The new law also carries a significantly higher penalty, with offenders at risk of being jailed for life.

Law scholar Simon Young Ngai-man, who specialises in state security laws at HKU, said the rationales behind the Beijing-imposed national security law and the Crimes Ordinance were also different.

Another major difference between Beijing’s national security law and the Crimes Ordinance, according to Young, is that the new law has a sharp focus on targeting calls for independence, foreign interference and what has been labelled acts of domestic terrorism.

Young also said the sedition offences in the Crimes Ordinance overlapped substantially with the incitement clauses found in Beijing’s national security law.

In 2003, the Hong Kong government tried to push a national security bill through the city’s legislature. The proposed legislation covered seven categories, including treason, secession, sedition and subversion. But the bill was shelved after more than 500,000 people took to the streets on July 1, the anniversary of the handover of Hong Kong back to Chinese rule.

Critics had argued against the need for Beijing to step in while Hong Kong already had existing laws to deal with offences viewed as endangering national security by Beijing.

But Lau Siu-kai, vice-chairman of the Chinese Association of Hong Kong and Macau Studies, a semi-official think tank, said the older law did not address Beijing’s concerns precisely enough because it was designed to shield the British monarchy and government. The colonial-era laws might also no longer be accepted by the city’s courts, he added, prompting Beijing’s move to impose the new legislation.

However, Johannes Chan Man-mun, an HKU human rights law scholar who has warned against the implementation of any new national security law, said Tam’s prosecution proved his point.

“I have been saying for decades that there is no need for a national security law as everything required under Article 23 has already been covered by our existing law, and this is another incident to confirm what I have said,” he said.

In fact, he added, the archaic law under which Tam was charged was even more extensive than the national security law Beijing put in place because the older legislation did not require prosecutors to prove any acts of subversion or secession.

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What is the Basic Law of Hong Kong?

What is the Basic Law of Hong Kong?

Why do critics say the sedition offences are problematic?

Human rights advocates and lawyers have argued that the colonial-era law has since gone out of date, with its restriction on free speech running contrary to the safeguards on human rights the city has put in place over the years.

“This law was enacted almost 100 years ago. If we still use this standard to evaluate it, I believe it will be very hard for society today to accept it,” said HKU’s Chan, who called the sedition offence’s scope “too broad”.

Eric Cheung Tat-ming, principal law lecturer at HKU, also said the law’s language was difficult to understand, leaving room for many forms of speech to fall foul of the offence.

But former director of public prosecutions Grenville Cross argued that a law should not be dismissed out of hand just because it was from the colonial era.

“The fact that a particular law has survived for a long time is often a mark of its utility, not an indication that it is antiquated,” Cross said.

Former Bar Association chairman Ronny Tong Ka-wah, now an adviser on Hong Kong leader Carrie Lam Cheng Yuet-ngor’s Executive Council, noted that the UN’s international covenant allowed laws to limit some types of speech.

Who was targeted under the law?

The sedition offences were most notably cited by the colonial government in dealing with the waves of riots in 1967, when pro-communist forces and their supporters launched mass strikes and violent demonstrations against the colonial government that claimed 51 lives.

In 1952, the editor of Ta Kung Pao, a pro-Beijing newspaper, and its printer and publisher were charged with seditious publication over a series of articles and editorials criticising the Hong Kong government’s decision to block a mainland group from visiting victims of a squatter fire.

However, the law had not been used for at least four decades until this year, said Margaret Ng Ngoi-yee, a barrister and a former opposition lawmaker.

Before Tam, the activist, was charged on Tuesday, opposition district councillor Cheng Lai-king was arrested in March for sedition under the same colonial-era law for reposting materials online criticising a police officer for shooting a journalist and calling for “an eye for an eye”.