Yuen Long, a quiet residential area close to the Chinese border, has become the unlikely next battleground of Hong Kong’s protest movement.

Over the last seven weeks, demonstrators have planned rallies across the territory – in parks, along main roads, in the airport and outside government offices – calling for the withdrawal of an extradition bill and making other political demands. But Yuen Long, known as one of the more remote, isolated areas in the north-west, had never been on the agenda.

On Sunday that changed. Commuters returning from dinner, going to meet friends or some coming back from the pro-democracy rally in central Hong Kong, pulled into the mass transit station to find dozens of men in white T-shirts waiting for them. They were masked, armed with rattan rods and other weapons.

Over the next half hour, passengers were chased, punched and lashed. Some frantically dialled emergency services but got no response. Police arrived after the assailants had gone and left before the men came back a second time. At least 45 people were sent to hospital.

The attack stunned locals, and many suspected the involvement of organised crime groups, the triads, which are active in the villages surrounding Yuen Long. Opposition lawmakers accused authorities of colluding with organised crime groups. Police arrested 12 people in connection with the attack, some with triad backgrounds.

I’m scared. I think all Hongkongers are scared Mei

In the immediate aftermath Yuen Long became a ghost town: shops across the main strip were closed and the streets were empty. Some locals rented hotel rooms out of town for a few nights.

As businesses gradually returned to normal, residents began bracing themselves for clashes between protesters, the police and suspected gangsters this weekend, as as previously planned demonstration in Kowloon, a commercial and residential district closer to central Hong Kong, on Saturday, was cancelled in order to hold a march in Yuen Long.

Facebook Twitter Pinterest A street march is planned for Saturday in Yuen Long. Photograph: An Rong Xu/The Guardian

“Before the terrorist attack, none of the people are coming to Yuen Long for protest,” said Max Chung, 39, a resident of Yuen Long who submitted paperwork to the police for the march this weekend. “We have our local issues, our local problems but it hasn’t really been that serious until the attacks. That’s why I was so surprised the villagers are so against the movement.”

Quick guide What are the Hong Kong protests about? Show Hide Why are people protesting? The protests were triggered by a controversial bill that would have allowed extraditions to mainland China, where the Communist party controls the courts, but have since evolved into a broader pro-democracy movement. Public anger – fuelled by the aggressive tactics used by the police against demonstrators – has collided with years of frustration over worsening inequality and the cost of living in one of the world's most expensive, densely populated cities. The protest movement was given fresh impetus on 21 July when gangs of men attacked protesters and commuters at a mass transit station – while authorities seemingly did little to intervene. Underlying the movement is a push for full democracy in the city, whose leader is chosen by a committee dominated by a pro-Beijing establishment rather than by direct elections.

Protesters have vowed to keep their movement going until their core demands are met, such as the resignation of the city’s leader, Carrie Lam, an independent inquiry into police tactics, an amnesty for those arrested and a permanent withdrawal of the bill. Lam announced on 4 September that she was withdrawing the bill. Why were people so angry about the extradition bill? Beijing’s influence over Hong Kong has grown in recent years, as activists have been jailed and pro-democracy lawmakers disqualified from running or holding office. Independent booksellers have disappeared from the city, before reappearing in mainland China facing charges. Under the terms of the agreement by which the former British colony was returned to Chinese control in 1997, the semi-autonomous region was meant to maintain a “high degree of autonomy” through an independent judiciary, a free press and an open market economy, a framework known as “one country, two systems”. The extradition bill was seen as an attempt to undermine this and to give Beijing the ability to try pro-democracy activists under the judicial system of the mainland. How have the authorities responded? Beijing has issued increasingly shrill condemnations but has left it to the city's semi-autonomous government to deal with the situation. Meanwhile police have violently clashed directly with protesters, repeatedly firing teargas and rubber bullets. Beijing has ramped up its accusations that foreign countries are “fanning the fire” of unrest in the city. China’s top diplomat Yang Jiechi has ordered the US to “immediately stop interfering in Hong Kong affairs in any form”. Lily Kuo and Verna Yu in Hong Kong

Mei, who runs a noodle restaurant near the train station, had reopened its doors on Tuesday. At 7pm her restaurant was half full. “I’m scared. I think all Hongkongers are scared,” she said.

Aubree Au, 39, said she was shocked and horrified by news of the attack, and unable to sleep that night as she watched live updates of what was happening. “It was literally a gangster movie in real life,” she said. It continued to affect her. “It hurts,” she said. “I was terrified, knowing it was where I live.”

Facebook Twitter Pinterest The main commercial street in Yuen Long. Photograph: An Rong Xu/The Guardian

A former satellite town built by the British in the 1970s and used for years as farmland, Yuen Long is still largely conservative and dominated by village traditions, according to Chung. Pro-Beijing and pro-establishment sentiment is strong, especially in the villages where the triads are believed to have the support of local authorities and Chinese public security.

Yet residents are also frustrated by China’s encroachment into their lives. They blame Chinese tourists and parallel traders, who can reach Yuen Long in half an hour by bus from Shenzhen, for driving up real estate and living costs and crowding their streets.

Many in Yuen Long may agree with the demonstrators but choose not to say so, given how polarising the topic is, according to Fong, 60, a caretaker from the area. “A lot of people in Yuen Long do support the protesters, but they won’t say it out loud. There’s a large silent majority,” he said.

Fong has been supportive of his son who has been at the protests. Fong who says he is “incredibly angry” at what he believes is the obvious collusion between the police and the triads, plans to attend Saturday’s march. “I’m not scared,” he said. “Hong Kong people care about these things. We all know it matters.”

Facebook Twitter Pinterest Lennon Walls that were put up by Hong Kong residents at the Yuen Long MTR station. Photograph: An Rong Xu/The Guardian

Not all residents – from Yuen Long or elsewhere – may be as brave as Fong and his son. This time protesters fear the triads, whose presence is usually visible only through the scattering of mahjong parlours, brothels advertised as massage services, and other businesses in Yuen Long. “Normally, they don’t hurt people. They’re businesspeople,” said Fong.

Before the attack on Sunday, posters had circulated online warning people not to come to Yuen Long and set up Lennon Walls – named for the wall daubed with John Lennon-inspired political graffiti in Prague – or else they would have their hands and legs “chopped”. The heads of several Hong Kong universities issued a letter making the “strongest plea” to students not to attend.

“You will have angry young people from various parts of Hong Kong and the triads may gather from all over Hong Kong. I’m worried and ordinary people are worried,” said Joseph Cheng, a retired political science professor at the City University of Hong Kong.

Facebook Twitter Pinterest Kay To says people in Yuen Long are starting to think more about their safety after Sunday’s attack at the railway station. Photograph: An Rong Xu/The Guardian

Despite the warning, on Tuesday evening commuters passing through the same station where Sunday’s attack took place had covered the walls, columns and railings with photos of the victims, photos of the attackers posing with their weapons, hundreds of sticky notes, and signs that said “Take Back Yuen Long”. A group of volunteers who had heard about the wall online had come to guard passersby posting things there.

For Kay To, 29, a protester from Yuen Long, the attacks have made people in this normally peaceful district think about their safety, their role as citizens and the responsibilities of their leaders.

“They are starting to care,” he said. “In Yuen Long we never had a protest maybe in these 100 years. There have been a lot of first times for Yuen Long.”