An organiser of an unofficial legislative primary election in 2020 has been blamed in court by a former ally for nurturing “radical” and “irrational” activists and politicising community affairs in the years leading up to the largest national security law crackdown against the opposition in Hong Kong.

Former district councillor Andrew Chiu Ka-yin on Wednesday began giving evidence at West Kowloon Court against 16 of the 47 opposition figures contesting a joint subversion charge stemming from the unofficial primary in July 2020.

The defendant-turned-prosecution witness said that before an alleged subversive plot to seize a majority in the Legislative Council, former law professor Benny Tai Yiu-ting had already planned for the opposition to dominate the municipal-level district councils with a view to eventually control the legislature.

Former district councillor Andrew Chiu gave evidence at West Kowloon Court against the opposition figures.

Chiu, who was convenor of the now-disbanded Power for Democracy group and a former Democratic Party member, is the second coordinator of the primary to testify for prosecutors in the 90-day trial. Another coordinator, former legislator Au Nok-hin, previously gave evidence for 24 days.

He cited Tai’s tactical voting strategy, first proposed in 2017 in the hope of maximising the chances of opposition candidates in the District Council election two years later, to highlight what he saw as the jailed academic’s attempt to turn the municipal offices into political forums.

The court heard that the plan was for the opposition to win a majority of district council seats in order to gain additional ones on an election committee that picked the city’s chief executive, making it harder for Beijing to influence the race for the top job.

The plan was eventually abandoned, but the opposition still cruised to a crushing victory in the 2019 poll following months of anti-government protests.

“At that time, Tai had already taken steps to promote in the district councils his idea of putting up resistance [against the authorities] using constitutional [tools],” Chiu said.

He added that Tai had repeatedly categorised the district council as an agent of political change and caused a surge of inexperienced localist politicians in recent years.

But he said that instead of focusing on local and livelihood issues, many of the novices who won in the 2019 race took a “populist” approach after taking office, turning the councils into a battle ground rife with smear tactics and political mantras.

Mr Justice Andrew Chan Hing-wai, one of a trio of judges hearing the trial, questioned whether the witness agreed that the new batch of district councillors were more radical and irrational but less constructive because of Tai’s involvement.

“That is more or less the case,” Chiu replied.

The judge continued: “So in short, Tai wanted to copy the success of the district council election in the Legco election.”

“I won’t use the word copy,” the witness said. “After the big win, he had in fact jumped on the bandwagon and made use of his status as a great scholar and wordsmith to write articles and push for [the holding of the primary].”

Chiu appeared to have lost weight and hair when he took to the witness box on Wednesday.

Power for Democracy, founded in 2002 and a mediator among opposition candidates in previous district elections, had helped organise and fund the primary which prosecutors alleged was part of a conspiracy to subvert state power.

Chiu said he was barely involved in setting the agenda of the unofficial event after Tai first floated the idea in early 2020, as he was forced to take a break from politics because of a serious ear injury sustained during the social unrest the previous year.

Tai, Au and Chiu are among 31 defendants in the case who have pleaded guilty to the subversion charge or indicated they would admit liability.

The 47 opposition figures were accused of organisation or participation in the primary to ensure they won a majority in Legco so they could paralyse the government and force Carrie Lam Cheng Yuet-ngor, then the chief executive, to step down.

Former district councillor Choy Chak-hung and Luke Lai King-fai, ex-chief officer of Power for Democracy, have been named as co-conspirators during the course of the trial, although they have never been arrested in relation to the case. The court heard Lai had left Hong Kong in July 2021.