BEIJING—A Taiwanese human-rights activist pleaded guilty in a Chinese court to charges that he had plotted to overthrow Communist rule in China, a case seen as a marker of soured ties between Beijing and Taipei.

Lee Ming-che stood trial on Monday alongside an alleged Chinese accomplice, Peng Yuhua. Both faced charges of “subverting state power” through activities conducted mainly on social media, a municipal court in the central Chinese city of Yueyang said on its official microblog.

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