Choi, a transport worker and local resident, initially denied one count of possessing an offensive weapon in a public place, but opted to plead guilty just before his trial. The offence carries a maximum prison term of three years.

Magistrate Winnie Lau Yee-wan said the defendant’s act was an apparent threat to public safety, which guaranteed a deterrent sentence.

“It was sheer luck that no one was injured,” she said. “The defendant should have made a police report, instead of carrying a weapon and resorting to violence.”

She said that in a video widely shared online, the defendant, wearing white, was seen clutching the knife in his right hand and chasing protesters for about five seconds. The knife was not sheathed, and some protesters threw bottles and shouted at the defendant.

Defence counsel Lawrence Ng Yeung-yin said in mitigation his client was angered after witnessing his friend’s wife being harassed by protesters following a dispute.

He said Choi’s family felt threatened when, on September 23, a wall outside his home was painted with graffiti reading: “Here lives a murderer, a knife-wielding man.”

Ng also claimed the defendant had tried to turn himself in before he was arrested, but was told at Lantau North Police Station that he did not need to surrender “for the time being”.

But prosecutor Francis Yip Kim-ming said his team had been unable to find any officer at the station who talked to Choi and could confirm whether such a conversation took place. Ng later withdrew this claim.

In sentencing Choi, the magistrate imposed a starting point of 15 months in jail, but reduced it by five months to credit his guilty plea.

She granted a further reduction of one month, as she found that the defendant appeared truly remorseful and brave to accept liability for his crime.

As it happened: Hong Kong airport protesters go on rampage in MTR station

Separately, police on Tuesday arrested five Hong Kong men in connection with a firebomb attack at Kowloon Tong railway station in October.

Investigations showed a masked pair had hurled two petrol bombs into the MTR station and fled in two getaway cars just before 2.30pm on October 12. Two turnstiles were damaged in the incident.

There was no protest in the area at the time but an anti-government rally was being held in Tsim Sha Tsui.

After combing through security camera footage, police identified the suspects. Officers from Kowloon West regional crime unit arrested five men aged from 26 to 40 in a series of raids across the city on Tuesday.

Four of them, including deliveryman Tsang Yu-sang, 26, IT technician Cheng Wai-sum, 33, and businessmen Wong Hon-wai and Wong Kam-faat, aged 30 and 40 respectively, were charged in Kowloon City Court on Wednesday afternoon.

They faced a joint count of conspiracy to commit arson.

Tsang was additionally charged with possession of offensive weapons, possession of imitation firearms and possession of an instrument fit for unlawful purposes.

Court documents showed that officers seized five petrol bombs, two air guns, two foldable knives, five crowbars and three hammers in his home in Sau Mau Ping.

Acting principal magistrate Ada Yim Shun-yee adjourned the case to March 2 pending further police inquiries, including examination of the exhibits and a fingerprint analysis.

She ordered that Tsang be remanded in custody after prosecutors objected to his bail, but granted bail of HK$100,000 to Wong Hon-wai, and bail of HK$10,000 each to the other two men on condition they stay in Hong Kong and report to police three times a week.