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.