China has arrested a Taiwanese man on state security charges after he crossed from Hong Kong into the mainland and reportedly shared pictures of troops, renewing fears about arbitrary detention amid the ongoing political unrest in the financial hub.

Lee Meng-chu, a volunteer activity organiser in the small Taiwanese fishing community of Fangliao, went missing on August 20 when he entered Shenzhen, a border city which links Hong Kong to Southeast China.

Mr Lee, who studied in the US and is also known by the name Morrison, allegedly shared information about the ongoing Hong Kong protests on social media and sent Chen Ya-lin, the Fangliao mayor, a photo of Chinese troops massing equipment on the city’s border.

The Shenzhen police had previously told Mr Lee’s worried family that they had no record of him arriving in the city, despite local friends reporting that they had dinner together before he disappeared.

Last month the Taiwanese government also made official enquiries about his whereabouts but received no reply from China.

On Wednesday, Ma Xiaoguang, a spokesman for the Taiwan Affairs Office in Beijing, confirmed that he was being investigated after he “allegedly engaged in illegal activities that endanger state security.” Mr Ma did not elaborate further.