Beijing: Sweden's foreign ministry said it was taking "strong measures at a high political level" after bookseller Gui Minhai was removed from a Chinese train by police while travelling with Swedish diplomats.
Gui, a Swedish citizen, made international headlines in 2015 when he was abducted from Thailand by Chinese police and brought to mainland China, amid a swoop on five Hong Kong book publishers.
In October, Gui was released from a Chinese prison after serving a sentence for an old traffic offence, but was yet to leave China.
On Saturday he was travelling by train to Beijing with two Swedish consular officials when the men, reported to be plainclothes police, boarded the train and removed him.
Dissident poet Bei Ling, one of the founders of the Independent Chinese PEN Centre and a friend, told the South China Morning Post that Gui may have been visiting the Swedish embassy to finish a new passport application.
The bookseller's daughter Angela Gui told Radio Sweden she was worried about his health. She told The New York Times a group of 10 men had "grabbed him from the train and took him away".
Her father had been travelling to Beijing to attend a medical examination for a neurological condition, she said.
Swedish foreign ministry spokesman Patric Nilsson confirmed to Radio Sweden that Gui had been taken away.
"This event will be handled with the utmost seriousness," he said.
The bookseller abductions in 2015 created fear among Hong Kong citizens that China was disregarding Hong Kong's separate legal system - in addition to Gui, one other bookseller was believed to have been directly taken from Hong Kong, while another three were detained on a trip to Shenzhen.
The five men, based in Hong Kong, had published and distributed hundreds of books specialising in political gossip and scandal about the Chinese leadership.
They later appeared on Chinese television claiming they had returned to mainland China voluntarily.
China director at Human Rights Watch Sophie Richardson said the second abduction in the presence of Swedish diplomats was "shocking".
"An enforced disappearance; all governments, not just Sweden, should be increasingly concerned about citizens in China," she wrote on social media.
Foreign Minister Margot Wallstrom told Swedish media she had summoned China's ambassador to demand more information, but wouldn't comment further "out of consideration for" Gui.
