Sweden’s foreign minister has condemned China’s “brutal” detention of a Hong Kong bookseller who irked Beijing with his tabloid-style stories about the Communist party elite.

Gui Minhai, a China-born Swedish citizen, was picked up by plainclothes agents on 20 January as he travelled on a train to Beijing with two Swedish diplomats. Supporters said he had been traveling to the Chinese capital for a medical examination.

Gui had spent much of the previous two years in custody in east China after his suspected rendition from his holiday home in Thailand. Until his second disappearance, hopes had been rising among supporters for his release.

In a statement Sweden’s foreign minister, Margot Wallstrom, said: “The continued detention of the Swedish citizen Gui Minhai in China is a very serious matter.”

Wallstrom labelled the action against Gui and the two diplomats as a “brutal intervention”: “China’s actions were in contravention of basic international rules on consular support.

“We demand that our citizen be given the opportunity to meet Swedish diplomatic and medical staff, and that he be released so that he can be reunited with his daughter and family.”

The statement signals a marked escalation from Sweden which has been publicly cautious over the case ever since Gui first went missing in October 2015.

On Sunday the publisher’s daughter, Angela Gui, told the Guardian that her father’s whereabouts remained a mystery.