"We expect the immediate release of our fellow citizen, and that he be given the opportunity to meet Swedish diplomatic and medical staff," the statement said.

A spokeswoman for China's Foreign Minister repeatedly declined to provide details on Guo's case during a press briefing on Tuesday.

"All foreigners in China, including diplomats in foreign embassies and consulates, must not violate international law or Chinese laws," said Hua Chunying. "That's common sense and a basic principle."

'I have not heard anything'

Gui's daughter Angela said her father had been diagnosed with the progressive neurodegenerative disease known as ALS.

She said he had been five hours into his trip to Beijing from his home in Ningbo when about 10 men in plainclothes had boarded the train, saying they were from the police.

The journey time between the two cities by rail is typically around 8 hours.

Gui was one of five Hong Kong-based booksellers who mysteriously vanished in late 2015 only to resurface in Chinese police custody. All were involved in publishing political gossip about China's Communist Party leadership.

The Swedish citizen was eventually released from custody after making a public confession on state broadcaster CCTV. His daughter said, despite his release, he remained under close police surveillance.

Since Xi came to power in late 2012, China has rounded up hundreds of activists and lawyers nationwide in what many analysts call the worst crackdown on human rights advocates in decades.