“If you see attackers, you must be armed with basic skills to save your own lives,” Henry Chong, a sports therapist and karate expert, told his first self-defence class.

The class, which took place two days after the attack on unarmed commuters in the rural town of Yuen Long , was attended by mostly women aged 20-60.

Hong Kong, reputed as one of the world’s safest cities, has been reeling from the attack at Yuen Long on Sunday evening, when men dressed in white rushed into an out-of-town station and used canes and wooden rods to indiscriminately beat passengers, among them protesters returning from a mass protest on Hong Kong island. At least 45 were injured.

The incident caused widespread anger, partly because there were no arrests at the scene. Hong Kong’s leader, Carrie Lam, vehemently denied accusations that the Hong Kong police colluded with the thugs in retaliation for the street protests. Police this week arrested 12 for unlawful assembly.

Within days of the attack, free self defence classes were springing up across Hong Kong. Many feel terrified and are struggling to come to terms with the violent attack that was so untypical of their hometown.

“Things that have happened in the past month seem so surreal; I can hardly believe this is Hong Kong,” said Jessica Chu, a 45-year-old full-time mother at Chong’s class. “The attack has created fear in all of us. Our basic sense of security has been ruined.”

The events in Hong Kong don’t look good lately … everyone needs to know how to defend themselves Eldad Jaeger

Hong Kong has been rocked by its largest political crisis in decades since 9 June, with millions thronging the streets to protest marching against the proposed extradition law, which many fear could result in government critics being sent to China to stand trial. The protests, which have ended in violent clashes in the past two weekends, have since morphed into a broader movement for universal suffrage.

Some say they learn self defence because they can no longer trust the police force – once recognised as the most professional in Asia – to protect them any more.

On the night of the Yuen Long attack, the police only arrived at the station after the assailants had left. Later, large numbers of men in white entered a nearby village and metal rods were found there, but police made no arrests and some walked out of the village in full view of many police officers.

“Hong Kong doesn’t feel safe any more. They didn’t immediately arrest the thugs and the officials were obviously covering for them. When the police fail to protect us what choice do we have but to defend ourselves?” said Rose Chan, a woman in her 40s who attended Chong’s class.

Hong Kongers attend free self-defence classes in a park. Photograph: Verna Yu/The Guardian

Eldad Jaeger, a judo instructor and a former member of the Israeli Defence Force, said he felt compelled to offer free classes. In a two-hour session in a park, he taught defence against punches and stick attacks to around 40 people ranging from age 12 to 40.

“The events in Hong Kong don’t look good lately … everyone needs to know how to defend themselves,” he said.

A 40-year-old mother who declined to give her name said she brought along her 12-year-old son because he goes to school alone in the underground railway.

“You never know what might happen. If there is an attack no one can escape,” she said.

“I am angered by the fact that police weren’t doing anything when people were attacked,” said Sonny Tsang, 26, an IT professional at the class. “The most dreadful thing is the collusion between police and gangsters. It’s beyond condoning, it’s a collaboration.”

But people learning self-defence say they do not want to be cowed by the attack. Some insist they will go to future protests, including one in Yuen Long this weekend.

“Things are becoming more violent, so we need to protect ourselves, but I believe more and more people will go onto the streets,” said Belinda Chan, a stage actor.