A former Hong Kong political activist found guilty of rioting at Polytechnic University (PolyU) at the height of the 2019 anti-government protests was given a reduced jail term of three years and eight months as the judge factored in a three-year delay caused by the prosecution adding a last-minute charge.

The District Court also handed down jail terms of between one year and 13 months to four co-defendants, including a civil servant, who were earlier convicted of perverting the course of justice for attempting to help protesters flee from police.

Three others who pleaded guilty to the same charge before the start of last year’s trial were jailed for between seven months and 10 months.

Passing sentence on Alvin Cheng Kam-mun, a former assistant to ex-legislator Cheng Chung-tai, on Saturday Deputy Judge Andy Cheng Lim-chi said rioting was a serious offence which warranted a deterrent sentence as he adopted a starting point of four years and six months.

“The scale of the PolyU riot was definitely not small, involving the use of Molotov cocktails, which can cause massive destruction,” he said.

“There was no evidence to show that the defendant was a leader of this riot. A sentencing discount should also be given as the defendant has suffered a delay in prosecution … which may increase his stress and anxiety.”

The court heard that his emergence from a sewer near the scene was proof of his participation in the violence at PolyU in November 2019. He was seen coming out of an underground sewage chamber 600 metres (1,968 feet) from the campus three days after police barricaded it.

The judge sentenced Cheng to three years and eight months in prison, saying the jail term was reduced because the prosecution only brought the new riot charge against him in late 2022, three years after he was arrested in 2019.

The new charge was brought against him when he was in the middle of a trial for perverting the course of justice, for which he was later acquitted, the judge added.

“The prosecution should have brought the riot charge earlier but only added the new charge when it had finished its evidence in chief. Due to the delay in prosecution, there is a sentencing reduction for the defendant.”

The demonstrations in and around PolyU marked one of the most intense episodes of the unrest, sparked by a now-withdrawn extradition bill.

Protesters began occupying the university in Hung Hom on November 11 that year, fortifying its entrances and battling police.

The force sealed off the university’s entrances on the night of November 17 and cornered more than 1,000 people, prompting some to search for escape routes, such as the sewage tunnels.

The court heard police intercepted driver Ip Ka-ki, 29, and decoration worker Ho Chi-ho, 30, on the morning of November 20 after spotting them loitering at Oi Man Estate in Ho Man Tin.

Messages on Ip’s mobile phone revealed plans to drive protesters to safety.

The information enabled officers to identify a manhole near the Royal Peninsula housing estate which protesters used to escape from PolyU.

Wearing a head flashlight and a gas mask, former activist Cheng, 34, was seen climbing out of the tunnel with the help of Chan Kai-yin, 35, an administrative officer of the Home Affairs Department, and jobless man Wong Yat-to, 27.

The judge earlier concluded the “only irresistible inference” was that Cheng came from the besieged campus, as nobody would enter the sewage tunnels unless they had been cornered or for work purposes.

Stage lighting technician Fung Sze-yui, 32, advertising director Lam Wing-man, 33, and salesman Leung Cho-kwong, 26, were seen in two nearby getaway cars.

Ip, Wong and Leung pleaded guilty last year to perverting the course of justice, while Ho, Chan, Fung and Lam were convicted of the same charge in January.

The judge sentenced the trio who pleaded guilty to jail terms of seven to 10 months and the four others to between one year and 13 months in prison, saying the defendants’ offence was a one-off to assist those trapped on the campus.

He said anybody found inside PolyU after the police blockade on November 17 must have chosen to remain and resist law enforcement, adding that the four co-defendants must have knowingly gone to the scene in the hope of helping those who escaped.