An employee at the UK’s consulate in Hong Kong has been detained by mainland Chinese authorities on his way back to the city, his girlfriend has said.

Simon Cheng, 28, was returning from a trip in Shenzhen to his native Hong Kong on 8 August when his girlfriend, Li, stopped receiving communications from him.

A spokesman for the UK’s Foreign and Commonwealth Office said: “We are extremely concerned by reports that a member of our team has been detained returning to Hong Kong from Shenzhen … We are providing support to his family and seeking further information from authorities in Guangdong province and Hong Kong.”

Li said Cheng had messaged her just before he went silent. “Ready to pass through the border … pray for me,” he had written.

More than 10 days later, Li and Cheng’s family have not been able to get in touch with him. Li said Hong Kong immigration authorities had told her Cheng had been placed under “administrative detention” in mainland China in an unknown location and for unknown reasons.

The detention of Cheng, who works in the British consulate as a trade and investment officer for Scottish Development International, comes amid more than two months of pro-democracy protests in Hong Kong that have threatened Beijing’s authority over the city.

In a separate development on Tuesday, two police officers were arrested after video emerged of them beating an older man on a hospital trolley, heaping further pressure on a force already facing accusations of brutality.

The footage, recorded in late June, shows two uniformed officers assaulting the man with batons and holding a cloth over his mouth. There are no other people in the room as the officers abuse the patient over several minutes. Police said the man was under arrest at the time.

Hundreds of thousands of protesters dressed in black rally in Victoria Park in Hong Kong on Sunday. Photograph: Kevin On Man Lee/Penta Press/Rex/Shutterstock

Cheng’s detention was first reported by the Hong Kong news site HK01. According to Li, he regularly travels to mainland China for meetings and had gone there most recently for work.

Hong Kong immigration officials said in an emailed statement that they had contacted the city’s liaison office in Guangdong province as well as the Hong Kong government “to understand the situation” and provide further assistance to the family.

Hong Kong has been rocked by mass protests triggered by a bill that would allow the extradition of suspects to mainland China. Cheng’s case further underlines fears of a lack of transparency and fairness in the mainland Chinese judicial system and the possibility that Hongkongers may be detained for political reasons.

Quick Guide What are the Hong Kong protests about? Show 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

Under administrative detention, under which authorities can detain and interrogate people for a broad number of offences, Cheng can be held for up to 15 days without charge. Human rights advocates say torture and other forms of ill treatment are common under this type of detention.

Li said she was not aware of Cheng participating in the protests or expressed his position on the movement in any public forums. “As far as I know, he did not attend any of the protests, even the 1 million march. I am just worried about him,” she said.

Cheng, who studied political economy at the London School of Economics, described himself as a “social activist” on his Facebook page. In early June, just before the mass protests began, he posted a yellow sign with people holding umbrellas – possibly a reference to the 2014 pro-democracy “Umbrella movement” in Hong Kong – and the words: “It’s the spirit that defines us.”

Li, who met Cheng while he was studying in Taiwan – where she is from – has been calling and writing to British, Chinese, and Hong Kong officials for help. “He grew up to be a very caring, diligent and hardworking person,” Li said. “Now, he is missing and detained for no reason. This is deeply distressing for his family and friends.”

Li said she and Cheng had discussed marriage, and possibly moving back to Taiwan, but Cheng wanted to remain in Hong Kong. “He said he loves Hong Kong and wanted to sacrifice all he has for his motherland.”

More than a million Hong Kong residents defied a police ban and poured into the streets on Sunday in a peaceful march, calling for the withdrawal of the extradition bill and other demands.

Hong Kong’s chief executive, Carrie Lam, said on Tuesday she would set up “a platform for dialogue” to tackle complaints against the police in the government’s first attempt at conciliation in weeks.

A rally in Cheng’s name has been called for outside the British consulate on Wednesday.

In recent weeks, Beijing has taken an increasingly hard line against the protests, which it has described as an attempt to start a “colour revolution”. Twitter blocked almost 1,000 accounts associated with the Chinese government on Monday while Facebook also removed seven pages, three groups and five accounts that originated in China.

China has repeatedly warned Britain – the former colonial ruler of Hong Kong – against “interference”.

Chinese immigration officials have increased checks at the border between mainland China and Hong Kong, detaining people and checking their phones and other devices.

Additional reporting by Verna Yu