A Hong Kong publisher with Swedish citizenship was seized on a train to Beijing despite being accompanied by two Swedish diplomats, his family say.
Gui Minhai made headlines when he vanished in Thailand in 2015 and later resurfaced in China, along with other publishing colleagues from Hong Kong.
He was freed from custody in October and has been living in Ningbo.
His daughter Angela said he was travelling to the Swedish embassy to undergo a medical examination.
She leads a campaign from the UK to free her father and told Radio Sweden that her father had been travelling to see a Swedish doctor as he was showing symptoms of the neurological disease ALS.
However, a close friend told the South China Morning Post that Mr Gui was thought to have applied for a new Swedish passport and may have been going to the embassy to complete the application.
Mr Gui's current whereabouts remain unknown.
Sweden said the Chinese ambassador had been summoned over the incident.
Mr Gui's train from Ningbo, a city in eastern China, was suddenly boarded on Saturday by plainclothes officers who took him away, his daughter said, although she did not know any more details of the confrontation.
The New York Times – which first covered the story – said two diplomats from the Shanghai consulate had been accompanying Mr Gui, who has published books on China's leadership.
Ms Gui told the newspaper: "I just know that things have taken a very drastic turn for the worse."
"The Swedish government is fully aware of what happened," Sweden's foreign ministry said, without confirming the details.
"Firm actions have been taken at a high political level and we have been in contact with Chinese officials who have promised us immediate information about his condition."
In Beijing, Chinese foreign ministry spokeswoman Hua Chunying said she had no information about Mr Gui.
But she added: "Any foreigner in China, including officials of foreign diplomatic missions, must not contravene international or Chinese laws."
After Mr Gui vanished in Thailand there were allegations that he had been abducted by Chinese agents across international borders in an extrajudicial process.
Chinese officials, however, say Mr Gui and the four other men all went to China voluntarily.
He later confessed to being involved in a road traffic accident more than a decade earlier – a confession supporters say was forced.
In October 2017, Mr Gui was released from prison, but could not be found by his family. It has since emerged that he has been living in Ningbo.
Human rights groups believe the publishers are victims of a crackdown on dissent against China's ruling communist party.
The former British colony of Hong Kong, however, is supposed to have distinct legal autonomy under the terms of its handover to China in 1997.