They were among a group of suspects acquitted in Kenya last week.
Taiwan's foreign ministry says they were forcibly put on a China-bound plane and has demanded their release.
China has not responded in detail to the allegations, but has criticised Taiwan for not considering itself as part of "one China".
The incident comes as cross-strait relations are feared to be entering a rocky period, say the BBC's Cindy Sui in Taipei.
Taiwan said the eight were among 23 Taiwanese people who were accused in Kenya of being members of a telephone fraud ring, which also included Chinese nationals.
They were arrested in Kenya at the end of last year and charged with illegal entry and telecommunications fraud.
All of the Taiwanese, and some Chinese, were later acquitted.
The Taiwanese were detained when they went to a police station last week to retrieve their passports.
On Friday, eight of them were put on a plane by Chinese officials and sent to the mainland, despite a court order that would have kept them in Kenya, say Taiwanese officials.
Taiwan has demanded their release, as well as for the release of the remaining 15 Taiwanese still in Kenyan custody.
On Monday, Taiwan's foreign affairs ministry accused Chinese officials of "obstructions", including delaying the court order and preventing Taiwan's representative from reaching the acquitted.
It said China's actions amounted to an "uncivilised act of extrajudicial abduction" which represents a "gross violation of basic human rights."
In response to a reporter's question on the matter, China's foreign ministry spokesman Lu Kang said: "I might need further understanding of the exact details of the case, but in principle, countries which follow the 'one China' principle are worthy of approval."
Beijing has refused to have dialogue with Taiwan's incoming President Tsai Ing-wen unless she recognises the two sides as part of one country.
Chinese President Xi Jinping and Taiwan's current President Ma Ying-jeou held historic but largely symbolic talks in Singapore last November, the first between China and Taiwan's leaders in more than 60 years.
Taiwan has accused China of "extrajudicial abduction" after eight Taiwanese acquitted of fraud in Kenya were deported to mainland China.