Hong Kong activist Yang Kuang was freed last night after being arrested in a swoop on a Shenzhen bus terminus yesterday and held at a police station - the second time he has been detained on the mainland this month.
He later told reporters on the phone that Shenzhen authorities told him he had crossed the border illegally.
Mainland rights activist Liu Shasha, who had been travelling with Yang, said the 45-year-old had sneaked into the mainland by boat last Sunday.
He had paid a visit to his mother in Guangzhou before returning to Shenzhen by bus yesterday.
Liu said about 20 plainclothes and uniformed security and immigration officers were waiting for them at the bus terminus in Futian district. The officers ordered Yang to go with them.
Yang, who was born and who grew up in Guangzhou, was one of the eight Diaoyu activists who managed to evade the Japanese coastguard and landed on the disputed islands in the East China Sea in August.
Earlier this month, he was detained for 40 hours by Beijing authorities when he tried to visit Liu Xia , who has been confined to her home since her writer husband Liu Xiaobo won the Nobel Peace Prize in 2010.
Yang had lost his home-return permit and the authorities cancelled it later.
Describing the latest events, Liu Shasha said: "When I tried to make a phone call at the terminus, they grabbed my phone. Someone even tried to handcuff me but were stopped by Yang."
The pair were taken to a police station in the Tian'an area, where Liu was questioned alone for three hours. She managed to run out of the station and was met by a group of online supporters. Officers then let her go.
On Sunday, Yang posted a picture of himself at a Guangzhou subway station on his Facebook page, saying: "I'm on the mainland. I'm good."
He added that he had "crossed the sea" to the mainland for "three poor ladies", his sick mother; Liu Shasha, who was injured by security officers earlier this month; and Liu Xia.
Before Yang was freed, Liu Shasha said she had learned from an internet contact that Yang's illegal entry would be punished by "administrative detention".