Kenyan authorities last Friday deported eight of the Taiwanese to China rather than to Taiwan.
Taiwan has protested to China after eight Taiwanese cleared of charges by a Kenyan court were "illegally" flown to the mainland, officials said Monday.
Kenyan authorities in November 2014 arrested 28 Taiwanese and 49 other ethnic Chinese on charges of illegally entering country and involvement in a telecoms scam, the foreign ministry said in a statement.
It said 37 of the suspects including 23 Taiwanese were found not guilty by a Kenyan court on Tuesday last week.
But Kenyan authorities last Friday deported eight of the Taiwanese to China rather than to Taiwan in response to pressure from Beijing, the ministry said.
The mainland used "technical methods" to delay news of the verdict reaching the relevant Taiwanese diplomat based in South Africa, it said.
"By the time our official rushed to the airport, the eight Taiwan citizens had been forcefully taken to a passenger plane of China Southern Airlines and sent to the mainland."
It denounced the move as "illegal" and "uncivilised", saying that the fundamental human rights of the eight had been infringed.
"The foreign ministry demands that the mainland immediately send the eight people back to Taiwan," it said, also calling on Kenyan authorities to free the other 15 Taiwanese cleared of the charges.
In reply to questions in parliament, Shih Hui-fen, deputy minister of Taiwan's Mainland Affairs Council said the protest was filed to Beijing at midnight last Friday.
"This has not only harmed the fundamental human rights (of the eight), but has hurt Taiwan people's feelings and has severe negative impacts on ties between the two sides," Shih said.
Concern has been growing about China's overseas activities since five Hong Kong-based booksellers went missing last year and ended up on the mainland.
Three went missing on the mainland, one in Thailand and one in Hong Kong itself — prompting accusations that Chinese law enforcement agents were operating illegally in the semi-autonomous city.
Taiwan and China, which split in 1949 after a civil war, signed a joint crime-fighting and judicial assistance agreement in 2009 amid improving ties.
But Beijing still sees the island as part of its territory awaiting reunification, by force if necessary.