An ailing Hong Kong bookseller detained in China should be released immediately and allowed to leave the country if he wishes, rights groups said as a growing number of European officials joined the call for Beijing to set him free.

Gui Minhai, who is a dual Chinese and Swedish citizen, was reportedly detained over the weekend while travelling alongside two Swedish diplomats, to seek medical help in Beijing.

In a press conference in the Chinese capital on Wednesday, Hans Dietmar Schweisgut, the European Union's ambassador to China, urged the government "to immediately release" 53-year-old Gui and "allow him to reunite with his family, to get consular support and to get medical support".

{articleGUID}

Amnesty International called for Gui's "unconditional release", adding that he should also be allowed to leave China at any time.

"Under Chinese laws, there is no legal justification that prevents Gui" from departing, Nicholas Bequelin, Amnesty International's East Asia Regional Director, told Al Jazeera.

He said Gui's detention "has been marked by egregious and brazen violations of human rights and international law".

Since Gui's disappearance in 2015, Chinese authorities "have demonstrated that they are not concerned with legal processes, and that they intend to act in any way for political motives", he said.

Mysterious cases of missing people

Gui had disappeared while on holiday in Thailand in October 2015.

He was one of five people to have gone missing that year from a Hong Kong bookstore and an affiliated publishing company, which specialises in publications critical of China's ruling Communist Party leaders.

At that time, Gui's family suspected he was abducted because of his work in Hong Kong, a special administrative region under China.

Three months after his mysterious disappearance, Gui turned up in China claiming on state media that he voluntarily turned himself in to answer to a drunk-driving incident in 2003, which reportedly resulted in a death of a student.

He was later sentenced to two years in prison. The other individuals later reappeared in Hong Kong.

Gui's detention and the disappearances of others prompted fears that Chinese authorities have been using tactics that infringe on Hong Kong's legal system.

In October 2017, China announced Gui's release, but he was not allowed to leave the country, and his movement was reportedly restricted within his hometown of Ningbo.

'Shocking' detention

On Wednesday, Human Rights Watch said the Chinese government has no "legal and legitimate explanation" to justify Gui's continued detention.

"He should be released immediately," Patrick Poon, an Amnesty researcher based in Hong Kong, told Al Jazeera, describing the manner of Gui's latest detention as "shocking".

"It's the first time ever that we have seen such a situation, wherein a person is accompanied by diplomats, and still being taken away by police," he said.

"That's why it's been a really big news for many" in Hong Kong, Poon said.

Gui's daughter, Angela, who is based in the UK, had told reporters that her father was showing symptoms of the neurological disorder ALS, prompting him to seek medical help in Beijing.

On Tuesday, Margot Wallstrom, Sweden's foreign minister, said her country expects "the immediate release of our fellow citizen, and that he be given the opportunity to meet Swedish diplomatic and medical staff".

She also confirmed that Gui's detention took place, while being accompanied by Swedish diplomatic staff, "who were providing consular assistance to a Swedish citizen in need of medical care".

"This was perfectly in line with basic international rules giving us the right to provide our citizens with consular support," Wallstrom said, adding that the Swedish officials received assurance from China that Gui "has been free since his release", and that they are allowed to contact him.

In the last three days, Wallstrom had summoned China's ambassador to Sweden at least two times to discuss Gui's case.

In response, China's foreign ministry said on Tuesday that the case does not fall "under the purview" of its office, suggesting the issue is domestic in nature. Ministry spokesman Hua Chunying instead warned that "staff of foreign embassies and consulates, cannot violate international and Chinese law". She did not make a direct reference to the Swedish diplomats accompanying Gui.

The state-run Chinese website, Global Times also ran an editorial with a banner headline, "Western media has no right to interfere in China's judicial process", adding it wants to "manipulate" public opinion.

Meanwhile, in a statement, the literary group PEN Hong Kong urged Chinese authorities to disclose his whereabouts, and release him.

"Gui Minhai's disappearance is deeply disturbing. Gui is potentially suffering from a serious degenerative condition and requires medical assessment," said Salil Tripathi, Chair of PEN International's Writers in Prison Committee.

In July of 2017, Chinese dissident and Nobel Peace Prize laureate Liu Xiaobo, died in hospital, after he was placed under detention for years, igniting criticism against China's human rights record.