The head of Hong Kong’s biggest police officers association has said self-proclaimed peaceful protesters that assist radicals in committing violent acts, or simply stand and watch could also face legal consequences.

In an open letter to his members, Lam Chi-wai, chairman of the Junior Police Officers' Association, said while radicals should be condemned for their violence, protesters who called themselves “wo lei fei”, a Cantonese phrase meaning “peaceful, reasonable and non-violent”, were just as culpable.

“Rioters who are directly involved in the violence surely need to be responsible for their serious crimes, but a group of self-proclaimed peaceful, rational and non-violent followers cannot escape the blame,” Lam said.

Lam pointed out that peaceful protesters were using their bodies, umbrellas and other items to cover up for radicals who were assaulting people or vandalising businesses, cheering for them, helping them escape, or encouraging others to break the law online.

He added that some just stood and watched while radicals beat up people of different opinions, or damaged shops.

“I urge the media and citizens to stop calling them ‘wo, lei, fei’,” he wrote. “Their act is as inhuman and cold-blooded as those violent rioters … they are blatant accomplices.”

Lam warned that those peaceful protesters could face legal consequences.

“Your acts of assisting rioters to commit crime and escape, or obstructing police officers to execute duties are possibly unlawful,” Lam said. “One day you would need to bear criminal liability for what you have done.”

In the letter, Lam lashed out at radical protesters for their actions over the Christmas holidays, and accused them of attacking individual police officers to the extent they seemed to want to kill them.

The city has been embroiled in protests since June, triggered by an unpopular extradition bill that was withdrawn in September, but the demonstrations have evolved into an increasingly violent wider anti-government campaign.

Over Christmas, masked protesters targeted shopping centres in tourist areas, chanting slogans and heckling customers outside shops with mainland China links. Riot police used tear gas and pepper spray to disperse crowds.

Between Monday and Wednesday, 336 people were arrested during protests for offences such as illegal assembly, and assault occasioning actual bodily harm, according to the police.

In another letter to a group of mainland high school students from Chengdu on the same day, Lam thanked them for their support of Hong Kong police, and the support from the central government.

“It has been almost six months since the outbreak of a series of violent incidents. Hong Kong and the police force have been facing an unprecedented challenge,” he wrote. “The support and encouragement from the central government are like an injection of confidence for us to stamp out the violence … with your support, we can expect the day that Hong Kong restores to its prosperity and stability.”

Meanwhile, Melissa Kaye Pang, president of the Law Society of Hong Kong, speaking in a radio programme on Saturday emphasised the importance of the rule of law amidst the “radical social movement”.

She said any violence, regardless of motive, should not be encouraged.

“Differences in political opinions is one thing, arson and assaults are another thing. Differences in political views does not justify one’s unlawful acts,” she said.