Pro-democracy activists have launched a private prosecution in London against five British officers working for the Hong Kong police, alleging they have taken part in brutal actions against protesters.

The officers – who have not been named – occupy senior roles inside Hong Kong’s local police force. They are accused of torturing anti-government demonstrators, who have been protesting since June last year over an extradition bill and security law imposed by Beijing.

The case is being brought by the prominent pro-democracy campaigner Nathan Law and the Hong Kong Watch activist Luke de Pulford. Law fled Hong Kong last month and is now based in Britain. He has called on the Hong Kong public to provide evidence.

“In Hong Kong’s system, there are no mechanisms to hold the police officers who have abused the rights of the people accountable,” he said. “Therefore, it’s important that we have actions in the UK and impose a deterrent effect on human rights violators.”

In May the activists appealed for witnesses to come forward and to describe their experiences of police abuse. Some have done so, but many are too scared to give testimony, de Pulford said. Protesters have suffered physical and psychological harm, as well as sexual abuse in police custody, he alleged.

Those bringing the case have sifted through thousands of hours of protest footage. It includes video taken on 12 June, when police fired teargas and rubber bullets at demonstrators who had gathered outside Hong Kong’s legislative council building. Crowds fled inside the nearby Citic tower.

de Pulford said his team was looking for admissible evidence that linked the five British officers to fast-moving events on the ground. Some were part of the chain of command, while others played a direct role in directing violence against protesters, he claimed.

The officers joined the police force while Hong Kong was still under British rule, in the 1990s, and now occupy senior roles. About 5% of the police force is made up of officers with British passports, de Pulford estimated. If the case goes ahead, the accused officers may have to fly to London and give evidence in court, he said.

“We want to send a signal against impunity. Right now there is no possibility in Hong Kong for any kind of justice,” de Pulford said. “One police officer on a moped ran into a group of protesters. He was suspended for a week.”

“If the UK’s moral and legal commitment to upholding the rights of Hong Kong people cannot extend to making its own citizens answer for their crimes, it is no commitment at all,” he added.

The UK claims universal jurisdiction for torture. In a press statement the group said a private prosecution “of senior British officers in the jurisdiction of England and Wales under universal jurisdiction provisions” was one of the few remaining options for justice.

It added: “At virtually every major protest, officers have beaten defenceless activists and bystanders and deploy disproportionate force with impunity – including stamping on the heads of protesters already in custody, as well as using the technique of kneeling on the necks of detainees and employing methods which contravene international policing norms and violate human rights law.

“Heinous accounts of sexual assault and rape in police custody have added to overwhelming evidence of widespread cruel and inhuman treatment,” the statement said.

In press interviews, the British officers have defended their conduct. They have denied using excessive force or brutality and have said that many police officers have been injured in confrontations with violent crowds. The authorities have reacted with moderation in the face of large-scale civil disorder, they have said.