A prominent Hong Kong democracy activist on Tuesday became the first person to be charged with sedition since the city was returned to Chinese rule as authorities widened their pursuit of critics in the financial hub.

Democracy campaigner and radio DJ Tam Tak-chi appeared in court on Tuesday facing five counts of "uttering seditious words" under a rarely used colonial-era law.

His prosecution comes just two months after Beijing imposed a separate national security law on Hong Kong to stamp out pro-democracy unrest, a move that drew condemnation from many Western nations and UN rights experts.

China argued the sweeping new law was needed to plug a lack of national security legislation in the restless city.

But the case against Tam uses a colonial sedition law that has been on the books since 1938 and will deepen fears that speech is being increasingly criminalised in the semi-autonomous hub.

The South China Morning Post on Tuesday said it was the first sedition charge since Hong Kong was handed back to China by Britain in 1997.

Tam is just the latest in a line of government critics to face prosecution in the months since Hong Kong was rocked last year by huge and often violent pro-democracy protests.

Hong Kong's Chief Executive Carrie Lam speaking to the media on September 8. She said criticism of the government would not violate any legislation
Hong Kong's Chief Executive Carrie Lam speaking to the media on September 8. She said criticism of the government would not violate any legislation AFP / ISAAC LAWRENCE

He was arrested on Sunday by a new Hong Kong police unit formed to investigate national security crimes.

Officers said the 47-year-old was held for using words "inciting hatred, contempt against the government and causing discontent and dissatisfaction among the Hong Kong people".

Local media on Tuesday said Tam was denied bail and that prosecutors gave examples of "seditious slogans".

They included the popular protest slogans "Liberate Hong Kong. Revolution of our times", "Five demands, not one less" and "Disband the police force", as well as the less commonly heard phrase "Death to corrupted police families".

Hong Kong authorities have been at pains to convince a sceptical public that freedoms will not be curtailed, but a climate of fear is sweeping the city under the new law.

Hours before Tam appeared in court, city leader Carrie Lam said criticism of the government would not violate any legislation.

"I can frankly say that criticising the chief executive will not become a violation as stated in the laws," she said.

But, Lam said, "there are certain circumstances where the law restricts freedom of speech".

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