- Video report by ITV News Asia Correspondent Debi Edward
Protesters have gathered outside the British Hong Kong consulate after a staffer was arrested by Chinese authorities.
Simon Cheng Man-kit was given 15 days of "administrative detention" by China after he went missing crossing the border for a business trip to Shenzhen, local media reported.
When he failed to return to work the next day, his family, including his girlfriend Li, informed the police.
Li, who met Mr Cheng when they were both studying in the UK, told news outlet HK01 he had contacted her on WeChat and WhatsApp shortly before the border crossing.
He said: "Ready to pass through the border," before adding: "Pray for me."
A friend of Mr Cheng Man-kit said he believes the arrest is designed to scare people in Hong Kong following two months of sustained protests.
Speaking outside the Hong Kong consulate where Mr Cheng Man-kit works, he said there is "no reason" for his friend to be detained by China.
He added: "We all know what happened in Hong Kong for these two months and China is trying to detain some of us, trying to scare some of us not to fight against the Chinese authority.
"And that's why he's very unlucky to be detained by the Chinese authority."
Another person speaking outside the consulate agreed that Mr Cheng Man-kit's detention was designed to stop Hong Kong citizens protesting.
He said: "First, to tell Hong Kong protesters that they shouldn't be doing protests at all, you shouldn't be exercising your civil liberties or civil rights. You shouldn't be speaking bad about the Chinese government.
"It's one of the ways they try to intrude Hong Kong's autonomy and their freedom of speech."
The announcement of Mr Cheng Man-kit's arrest comes a day after Geng Shuang, a Chinese Foreign Ministry spokesman, said he was "not aware of the situation".
It comes amid months of intense protests surrounding mainland China's alleged interference in Hong Kong's domestic politics.
A Foreign and Commonwealth Office said on Tuesday: "We are concerned by reports that a member of our team has been detained while returning to Hong Kong from Shenzhen.
"We are providing support to their family and seeking further information from authorities in Guangdong province and Hong Kong."
Mr Cheng is a Hong Kong permanent resident who had studied in Taiwan and the UK and it is not clear whether he held a diplomatic passport or what documents he used to enter China, the HKFP reported.
The Scottish Government said it was concerned for Mr Cheng's welfare and was liaising with the Foreign Office.
A spokesperson added: "We are aware of this incident and we are concerned for Mr Cheng's welfare.
"We are liaising with the FCO, who are in contact with Mr Cheng's family, the authorities in Guangzhou and Hong Kong, and are seeking further information."