A worker at the UK's Hong Kong consulate has been detained at the border for allegedly violating the law, China has confirmed.
Media reports on Tuesday said Simon Cheng, who is thought to be from Hong Kong, went missing on 8 August during a business trip.
China's foreign ministry said Mr Cheng was detained at Shenzhen for 15 days.
The UK said it was "extremely concerned" and the embassy in Beijing was providing support to his family.
China said Mr Cheng, 28, had been detained for violating public security laws, although the foreign ministry gave no details of the alleged offence.
Following large-scale protests in Hong Kong, travellers have reported heightened security measures on the mainland side, with people passing through being subjected to police checks of their mobile phones.
The protests, now entering their third month, were sparked by a now-suspended extradition bill that would have allowed Hong Kong to send criminal suspects to China for trial.
A small group of protesters gathered outside the British consulate in support of Mr Cheng. Max Chung, a spokesman for the protesters, told Reuters the case had serious consequences for everybody in Hong Kong.
"We want to urge the UK government to step up and act now. Save Simon now," he said, adding that Mr Cheng had not taken part in anti-government demonstrations.
"Simon is a very good guy, and he's a smart guy - he finished his Masters at LSE (London School of Economics) - so I don't think he would do anything stupid."
China's foreign ministry spokesman Geng Shuang said they had made "stern representations" to the UK over comments made since the protests began in Hong Kong.
"We request they stop making these irresponsible statements, stop meddling in Hong Kong's affairs and stop interfering in China's internal affairs," he said.
He said Mr Cheng was a Chinese citizen and his detention was an internal affair.
BBC China correspondent Robin Brant said the detention was short compared to many sentences in China and that Mr Cheng should be due for release in the next 48 hours.
He said China insisted the detention was not a diplomatic incident, but he said the Foreign Office had likely been working out of the public spotlight over the last 13 days to find out what had happened to Mr Cheng and secure his release.
Earlier this month, Foreign Secretary Dominic Raab raised concerns about the treatment of protesters in Hong Kong, prompting China to accuse the UK of "colonial" attitudes.
According to local outlet HKFP, Mr Cheng is a trade and investment officer at the Scottish Development International section of the consulate.
It said on 8 August he travelled to a business event in Shenzhen in south-east China, which links Hong Kong to the mainland.
His girlfriend told HKFP that he planned to travel home the same day, but did not return. In online messages, he wrote that he was passing through the border, adding "pray for me".
Hong Kong police said they had inquired about Mr Cheng's whereabouts with the mainland authorities but "results are still pending".
Earlier this year, China also arrested two Canadians, accusing them of espionage after detaining them for several months. It followed the arrest in Canada of Huawei executive Meng Wanzhou on behalf of the US.