Beijing (CNN) A Swedish book publisher under house arrest in China was seized by plainclothes police on a train Saturday in front of diplomats, his daughter said, reigniting international criticism of Beijing's treatment of its critics.

Gui Minhai, who wrote and published numerous titles critical of the Communist leadership, was traveling to Beijing to be examined by a Swedish doctor at the embassy when he was taken, his daughter Angela told Radio Sweden on Monday.

Gui had been diagnosed with a progressive neurodegenerative disease known as ALS, his daughter said.

It's the second time in just over two years that Gui has been allegedly seized by Chinese agents. He first disappeared in late 2015 from his holiday home in Thailand.

Angela Gui, daughter of missing Hong Kong-based bookseller Gui Minhai, testifies before the U.S. Congressional Executive Commission on China.

"He was escorted on this train journey on Saturday by two diplomats and they were on the train for about five hours," Angela Gui said from Cambridge, England, where she is studying.

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