“This trial is illegal,” said Hsiao I-Min, who traveled to Yueyang with Mr. Lee’s wife, and is with a non-governmental organisation Judicial Reform Foundation. Mr. Lee “was forced to confess a false truth.”

A Taiwanese activist pleaded guilty on Monday to subverting state power in China’s first prosecution of a nonprofit worker on criminal charges since Beijing passed a law tightening controls over foreign non-governmental organizations.

A supporter said he had been forced to confess to crimes he hadn’t committed.

“I spread articles that maliciously attacked the Communist Party of China, China’s existing system and China’s government,” the activist, Lee Ming-che, told the court in the central Chinese city of Yueyang. Lee said he also organized people and wrote articles “intended to subvert the state’s power.”

Subversion of state power is a vaguely defined charge often used by authorities to muzzle dissent and imprison critics.

Mr. Lee’s wife, Lee Ching-yu, who was in Yueyang for the trial, had warned that he might be pressured into pleading guilty. China’s wide-ranging crackdown on civil society has featured a string of televised “confessions” believed to have been coerced from human rights activists accused of plots to overthrow the political system.

On Monday, his supporters blasted the legal process.

“This trial is illegal,” said Hsiao I-Min, who traveled to Yueyang with Mr. Lee’s wife, and is with the Taiwanese non-governmental organization Judicial Reform Foundation. Mr. Lee “was forced to confess a false truth.”

“Pursuing democracy and freedom is not a crime,” Hsiao continued. “Mr. Lee was accused by the Chinese government of discussing and spreading ideas about democracy from the West. We think this is a basic human right.”

Security was tight at the Yueyang City Intermediate People’s Court, with barricades on the streets, dozens of security personnel patrolling the perimeter and reporters ordered to leave the area.

Mr. Lee Ming-che (42), has conducted online lectures on Taiwan’s democratization and managed a fund for families of political prisoners in China. He cleared immigration in the semi-autonomous Chinese territory of Macau on March 19 but never showed for a planned meeting with a friend later that day.

Amnesty International and other rights organisations have called for his immediate release.

The new law says foreign NGOs must not endanger China’s national security and ethnic unity, and subjects nonprofit groups to close police supervision. It is seen as an attempt to clamp down on perceived threats to the ruling Communist Party’s control.

Relations between Taiwan and China have been near an all-time low since the election of Taiwanese President Tsai Ing-wen, whose Democratic Progressive Party has advocated Taiwan’s formal independence. China cut off contacts with Taiwan’s government in June, five months after Ms. Tsai was elected.

Mr. Lee’s co-defendant, Peng Yuhua, who is from mainland China, also pleaded guilty. Mr. Peng said he had founded an organization called Palm Flower Company to pressure China to accept a multiparty political system. Mr. Lee was his deputy in charge of education, Mr. Peng said.