A key prosecution witness in the trial of Jimmy Lai took the stand in a Hong Kong court on Wednesday, giving evidence that the UN’s special rapporteur has said could be tainted because of allegations of torture.

Andy Li, a computer programmer turned pro-democracy activist, gave evidence about his role in a crowdfunding campaign, Stand With Hong Kong, to rally support for the pro-democracy protests in 2019. The campaign, which ultimately raised more than $1.8m (£1.4m), placed advertisements in several newspapers including the Guardian, the Washington Post and the Australian.

Prosecutors allege that Lai, 76, was the mastermind and financier behind the crowdfunding campaign, part of the two charges that the former media mogul colluded with foreign forces in breach of the national security law. Lai has pleaded not guilty to the charges against him.

Lai’s supporters say Li’s evidence was obtained through torture and therefore should not be relied upon.

In January, after an appeal from Lai’s international legal team, the UN’s special rapporteur on torture, Alice Jill Edwards, wrote to the Chinese authorities expressing concern about Li’s testimony.

“I am deeply concerned that evidence that is expected to be presented against Jimmy Lai imminently may have been obtained as a result of torture or other unlawful treatment,” Edwards said. “An investigation into these allegations must be conducted immediately before any evidence is admitted into these present proceedings.”

The UN convention against torture, which China has ratified, prohibits the use of evidence obtained through torture in legal proceedings, unless those proceedings are against an alleged torturer.

Li was one of 12 Hong Kong activists who were caught trying to flee by speedboat to Taiwan in August 2020. He was held in a prison in Shenzhen, in southern China, for seven months for illegal border crossing before being returned to Hong Kong, where he was charged with conspiring with foreign powers.

According to an investigation published by the Washington Post last year, while Li was detained in Shenzhen screams could “consistently” be heard from his cell. The detainees were reportedly kept in solitary confinement with the lights constantly switched on.

Li pleaded guilty to the charge of conspiring with foreign powers and will be sentenced after Lai’s trial, which Lai’s supporters say further compromises Li’s evidence. It is thought he is being held in a psychiatric facility.

Caoilfhionn Gallagher KC, the head of Lai’s international legal team, wrote on X: “Andy Li should not be giving evidence at all. But remember, anything he does say is unreliable.”

Li previously volunteered for the Inter-Parliamentary Alliance on China, a pressure group focused on the actions of the Chinese government. Luke de Pulford, the group’s director, said: “Andy’s evidence shouldn’t be admissible on the basis of his alleged torture alone, but this case isn’t about the law. It’s a political persecution where Andy Li is tragic collateral.”

Lai’s domestic legal team, who are representing him in the national security trial and have no connection to the international team, reportedly did not object to Li’s presence in court on Wednesday or make any reference to his alleged mistreatment.