BEIJING >> A Hong Kong-based book publisher with Swedish citizenship who was secretly spirited to China and held in custody for two years, igniting international controversy, has disappeared again in dramatic fashion — snatched from a train bound for Beijing under the eyes of two Swedish diplomats.

The bookseller, Gui Minhai, became a symbol of the Chinese government’s determination to smother criticism from abroad when he was one of five Hong Kong booksellers who disappeared in 2015, and then resurfaced in China in police custody.

Gui, who vanished from a vacation home in Thailand, was accused by Chinese state news media outlets of publishing books that slurred Communist Party leaders. Months after his disappearance, he appeared on state television confessing culpability for a drunken driving death more than a decade earlier.

Gui was formally freed from detention last October. But he has been kept in China and forced to report regularly to police, his daughter, Angela Gui, said by telephone from Britain, where she is a graduate student.

She said he had been living in an apartment in Ningbo, an eastern coastal city, where she has been able to speak with him frequently using Skype.

Then on Saturday, Gui vanished again.

Gui was sitting on a train bound for Beijing, accompanied by two diplomats from the Swedish Consulate in Shanghai, Angela Gui said. As the train neared Beijing, plainclothes police officers boarded at a station and led Gui away. His daughter said she did not have details of what happened, and did not know whether he or the diplomats resisted.

“I just know that things have taken a very drastic turn for the worse,” said Angela Gui, who has led a campaign to win her father’s freedom.

“This group of about 10 men in plain clothes just came in and grabbed him from the train and took him away,” she said. “I presume it must have been quite a scene.”

Renewed detention of Gui could rekindle strains between China and Sweden and its European allies. The Swedish Ministry of Foreign Affairs did not give details of what happened on the train, but confirmed the episode was serious.

“The Swedish government is fully aware of what happened to Swedish citizen Gui Minhai on Saturday,” Katarina Byrenius Roslund, deputy director of the Press and News Department of the ministry, said by email. “These events have subsequently been handled with the utmost seriousness, and robust measures have been taken at senior political level.”

When he was taken off the train, Gui had been traveling to the Chinese capital for a medical examination at the Swedish Embassy, after he showed symptoms of amyotrophic lateral sclerosis, also called ALS. The disease attacks the brain and spinal cord, progressively impairing actions like walking, swallowing and using chopsticks.

Last year, Liu Xiaobo, the imprisoned Chinese dissident who won the Nobel Peace Prize, died under police guard in a Chinese hospital after he was belatedly found to have liver cancer.

“Gui Minhai is exhibiting symptoms of a serious neurological condition, symptoms that did not exist before he was taken into custody in October 2015,” said John Kamm, an American businessman and human rights advocate who has been working with Gui’s family to try to win his release to go abroad for treatment. “I pray we will not witness the death in prison of another person accused of political crimes.”

The Chinese Ministry of Foreign Affairs said that it “had no understanding of the situation you referred to,” and suggested asking another ministry. The Ministry of Public Security did not answer questions faxed on Monday.

Gui, 53, was born in eastern China, went to study in Sweden in 1988 and became a citizen of the country in 1992. But in recent years, he worked out of Hong Kong, where he became a co-owner of Mighty Current Media, a small publishing house.

Supporters call the detention of Gui and the four other Hong Kong booksellers a campaign by the Chinese government to shut down publishers of books offering unsparing criticism and also unflattering gossip about the party elite. Hong Kong, a former British colony attached to mainland China, became a center for producing Chinese-language books that were banned in China, but that mainland readers often bought and smuggled back.

The mysterious detentions magnified worries in Hong Kong that China has grown dismissive of the legal guarantees that are intended to protect the city, which was returned by the British in 1997, from interference by the mainland authorities.

But Gui’s latest disappearance suggests that at least some in the Chinese security forces remain unbowed by the criticisms of the secretive detentions. His daughter said she was unsure which arm of the Chinese security or police forces boarded the train. In recent months, he had visited the Swedish Consulate in Shanghai several times without incident, she said.

After Gui was taken away, Chinese officials told Swedish diplomats that he was suspected of sharing secret information with Swedish diplomats and of meeting them illegally. Angela Gui said she was baffled by what secrets her father could have known, and did not see how it could be unlawful for her father to meet a Swedish diplomat.

“Obviously, they’ve acknowledged that he’s a Swedish citizen,” Angela Gui said of the Chinese police. “How that is a crime is difficult to comprehend.”