Simon Cheng is back in Hong Kong, say relatives and friends, after weeks of detention in mainland China

A staff member at the British consulate in Hong Kong who was detained in mainland China has been released, his family said on Saturday, ending an ordeal that lasted more than two weeks.

“Simon [Cheng] has returned to Hong Kong. Thanks to everyone for your support!” said an online message from Rescue Simon Cheng Facebook page run by his family and friends. It said Cheng and his family needed “time and space” to rest and recover and would not take any interviews for the moment.

“Simon is released. Simon is safe,” said Max Chung, organiser of a rally earlier this week to urge the British government to step up efforts to free Cheng.

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Simon Cheng, 28, a trade and investment officer for Scottish Development International, travelled to Shenzhen, a city that borders Hong Kong, on the morning of 8 August. He disappeared after sending messages to his girlfriend as he was about to cross back over the border at about 10pm.

Police in Shenzhen said Cheng was released as scheduled after 15 days of administrative detention and had “confessed to his illegal acts”, the statement said, without providing further details. In recent years, many Chinese activists have been forced to speak against their will and confess to alleged crimes on state media.

Earlier this week, a Chinese foreign ministry spokesman said Cheng had violated China’s public security administration punishment law. He was later accused by Chinese state media of visiting prostitutes, although his friends and family have suggested they did not believe this. They said earlier that “the truth is in people’s hearts”. The accusation is often used by the Chinese authorities to smear the reputation of government critics.

Britain’s Foreign and Commonwealth Office said: “We welcome the release of Simon Cheng and are delighted that he can be reunited with his family. We will continue to provide support to them.

“Simon and his family have requested privacy and we would be grateful if this is respected.”

Facebook Twitter Pinterest Simon Cheng was detained in China Photograph: Simon Cheng

Cheng’s detention comes amid Hong Kong’s largest political crisis in decades. More than two months of non-stop pro-democracy protests have threatened Beijing’s authority over the city. Last Sunday, more than a million Hong Kong residents defied a police ban and poured into the streets in a peaceful march, calling for the withdrawal of a controversial extradition bill and making other demands.

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His friends and family did not know why he was detained. Cheng’s girlfriend, Li, said earlier he had not participated in pro-democracy protests or publicly expressed his position on the movement.

Reports have emerged that many Hong Kong residents have been interrogated upon entering mainland China, taken into rooms, their messages and photos on their phones and computers checked along with documents they carried.

China has a long history of arresting dissidents, activists and government critics on trumped up sex or financial crimes.