It was meant to be a quick trip over the border to see the dentist.

Alex Wong, 35, and his girlfriend took the Hong Kong subway to the end of the line close to the Chinese border and then walked to immigration, where they would cross into Shenzhen.

Wong had completed the trip a number of times to seek cheaper medical services in China – a common practice for many Hong Kongers – but this time he would miss his appointment as he spent 21 hours handcuffed and in in police detention.

He was not charged with any crime but Chinese police had decided that he would be needed to “assist in an investigation” into Hong Kong’s ongoing anti-government protests after immigration agents found photos on his phone at the border.

Wong described himself as a “peaceful, rational, non-violent person” who participated in protests as a volunteer first-aid worker – one of hundreds who help protesters with tear gas or abrasions after clashes with riot police.

Nevertheless, the questions came.

“They asked me what happened and what I did during the government demonstrations,” he tells The Independent a week later, back in Hong Kong.

Continuing protests rock Hong Kong: In pictures Show all 35 1 /35 Continuing protests rock Hong Kong: In pictures Continuing protests rock Hong Kong: In pictures Police officers point their guns towards pro-democracy protesters after a clash at a march in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures Pro-democracy protesters throw molotov cocktails during clashes with riot police in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures Demonstrators rally behind barricades ass they are shrouded in tear gas deployed by police in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures Police clash withpro-democracy protesters at a march in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures A police officer prepares to strike a protester as clashes erupt during a pro-democracy march in Hong Kong on August 24 AFP/Getty Continuing protests rock Hong Kong: In pictures A water cannon is fired by riot police to clear the street of demonstrators during a protest in Hong Kong on August 25 EPA Continuing protests rock Hong Kong: In pictures Pro-democracy protesters use a slingshot during clashes with riot police in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures Riot police fire tear gas to disperse pro-democracy protesters in Hong Kong on August 24 Reuters Continuing protests rock Hong Kong: In pictures A demonstrator throws a molotov cocktail as they clash with riot police during a protest in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures A demonstrator swings a tennis racket in an attempt to knock back a tear gas cannister thrown by riot police during a protest in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures A protester displays a Hong Kong Independence flag during a pro-democracy march in Hong Kong on August 24 EPA Continuing protests rock Hong Kong: In pictures Riot police tend to a detained demonstrator who has been injured during a protest in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures A demonstrator uses bamboo poles to build a barricade during a protest in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures Riot police clash with demonstrators during a protest in Hong Kong on August 24 Reuters Continuing protests rock Hong Kong: In pictures Protesters are hit by tear gas fired by the police in Hong Kong on August 24 AFP/Getty Continuing protests rock Hong Kong: In pictures Demonstrators shrouded in tear gas shield themselves behind barricades during a clash with riot police in Hong Kong on August 25 Getty Continuing protests rock Hong Kong: In pictures Riot police arrest a protester during a pro-democracy march in Hong Kong on August 24 EPA Continuing protests rock Hong Kong: In pictures Police clash with pro-democracy protesters at the Hong Kong international airport on August 13 AFP/Getty Continuing protests rock Hong Kong: In pictures Protesters fall back as they clash with police during a protest at Kowloon Bay in Hong Kong on August 24 AFP/Getty Continuing protests rock Hong Kong: In pictures Police clash with pro-democracy protesters at the Hong Kong international airport on August 13 Reuters Continuing protests rock Hong Kong: In pictures Riot police move to disperse protesters during a clash at the Hong Kong international airport on August 13 Reuters Continuing protests rock Hong Kong: In pictures Commuters riding in a bus drive past protesters in Hong Kong's Kowloon Bay on August 24 AFP/Getty Continuing protests rock Hong Kong: In pictures Protester hold umbrellas as they take part in a pro-democracy march in Hong Kong on August 24 EPA Continuing protests rock Hong Kong: In pictures Riot police detain a demonstrator as they clash during a protest in Hong Kong on August 24 Reuters Continuing protests rock Hong Kong: In pictures Pro-democracy protesters block the entrance to the airport terminals after clashes with police at the Hong Kong international airport on August 13 AFP/Getty Continuing protests rock Hong Kong: In pictures Riot police spray pepper spray as shield themselves by a police van during a clash with pro-democracy protesters at the Hong Kong international airport on August 13 Reuters Continuing protests rock Hong Kong: In pictures Riot police use pepper spray to disperse protesters during a demonstration at the Hong Kong international airport on August 13 Reuters Continuing protests rock Hong Kong: In pictures Demonstrators shrouded in tear gas shield themselves behind barricades during a clash with riot police in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures Medical professionals gather to protest police brutality at the Queen Elizabeth Hospital in Hong Kong on August 13 Getty Images Continuing protests rock Hong Kong: In pictures Police detain a pro-democracy protester at the Hong Kong international airport on August 13 AFP/Getty Continuing protests rock Hong Kong: In pictures A protester is halted by police during an occupation at the Hong Kong international airport on August 13 AP Continuing protests rock Hong Kong: In pictures Police clash with pro-democracy protesters at the Hong Kong international airport on August 13 AFP/Getty Continuing protests rock Hong Kong: In pictures Police stand guard at the entrance of the departure terminals at the Hong Kong international airport on August 13 AFP/Getty Continuing protests rock Hong Kong: In pictures Protesters continue their occupation at the Hong Kong international airport on August 14 AP Continuing protests rock Hong Kong: In pictures Travellers read placards and posters left by pro-democracy protesters at the Hong Kong international airport on August 14 AP

He says immigration agents looked through his photos, which they tried to download, and also at conversations on his social media apps.

After being question by border police he was taken away to a police station where he was questioned for another eight hours in a special interrogation chair that restricted his hands and feet.

“I asked for a lawyer but they said no. Even [when] I wanted to call my family, they said no,” he says.

Wong’s case is an extreme but increasingly phenomenon for residents of Hong Kong, a former British colony, as they attempt to cross the border in recent weeks.

While the Asian financial city returned to Chinese sovereignty in 1997, it was promised a special semi-autonomous status for 50 years and for now maintains a British-style legal system under the “one country, two systems” agreement.

Protests started in early June when residents thought those rights were being eroded by a legislative bill that would allow them to stand trial on a number of criminal charges in mainland China, a place known for political prosecutions, a high conviction rate.

Unlike Hong Kong, it also maintains capital punishment.

The bill was meant to bring the two jurisdictions closer together, according to Hong Kong leader Carrie Lam, but it has become a major wedge between the local government and hundreds of thousands of residents.

Now it seems many fears about Hong Kong’s relationship with China, and its very different legal system, are coming true even after the legislative bill was suspended with border detentions and interrogations of ordinary people appearing to be commonplace.

Beijing considers the protests to be inching towards “terrorism” while Chinese state media have also described it as a violent “colour revolution”, which may be one reason why it may be loathe for images and videos to circumvent the Great Firewall.

In the most high profile case to date, Simon Cheng, a Scottish trade and investment officer at the UK consulate in Hong Kong, was detained as he was passing through an immigration checkpoint as he returned from Shenzhen.

He spent 15 days in “administrative detention” in China, allegedly for soliciting prostitution, according to Chinese state media.

This is a vague legal manoeuvre often used to hold people without charge, and does not require Chinese policy to notify Hong Kong authorities that a resident has been detained.

It has become common knowledge that you don’t bring your mobile phone to mainland China. Lam Cheuk-ting, Democratic Party legislator

Cheng’s case is even more concerning as he was detained at a train station on Hong Kong soil, according to Hong Kong Free Press.

The new express rail link sees passengers go through Chinese customs on the Hong Kong end of the station, meaning mainland law applies once they pass the checkpoint.

The immigration arrangement was controversial when it first announced and fears appear to be coming true, according to Jason Y Ng, convenor of the Progressive Lawyers Group in Hong Kong, with searches and detention now commonplace.

“The British consulate staffer was a prime example of the danger and his case has made the danger real,” says Ng.

Hong Kong police point guns at protesters

He says the group was afraid of “invasion of privacy, denial of entry, possible arrest on the mainland, and installation of Trojan Horse apps” on residents’ phones which can relay information back to security services long after the interrogation is over.

Democratic Party legislator Lam Cheuk-ting, who was contacted to help with Wong’s case by his family and girlfriend, says he had personally been messaged about half a dozen cases of border detentions by concerned family and friends.

Other legislators have also been contacted as well, he says, but suspected there were many more cases of residents who had chosen to remain silent.

"For most of them the reason for their detention is the mainland [immigration] officers found some photos or footage of the protests of Hong Kong on their mobile phone,” he says.

“After the [recent] movement started, it has become very common. They check the mobile phones of the youngsters who wear black T-shirts to everyone [else],

“It has become common knowledge that you don’t bring your mobile phone to mainland China.”