TAIPEI (Reuters) - China must not conduct a “black box” hearing against a Taiwan activist arrested on suspicion of subversion, but openly judge his case and disclose all the evidence, Taiwan’s ruling party said on Saturday.

Activist Li Ming-che, a community college worker known for supporting human rights, went missing in China on March 19, straining already poor relations between China and Taiwan.

China’s official Xinhua news agency reported on Friday that China’s Taiwan Affairs Office said Li had been formally arrested by state security authorities in the southern province of Hunan on suspicion of subversion of state power.

“The Chinese authorities should disclose all the evidentiary information and openly hear the case with the attitude of a civilized state, rather than dealing with the case in a black box operation,” Taiwan’s independence-leaning Democratic Progressive Party (DPP) said in a statement.

China’s state-run Xinhua said that after being interrogated Li had confessed to engaging in activities to harm state security. It said “judicial authorities will handle the case in accordance with the law”.

Taiwan’s Mainland Affairs Council, which handles China affairs, has said it does not agree with the alleged charges against Li, describing them as vague and unconvincing.

China’s judicial system must protect Li’s legal rights and Beijing should cooperate with Taiwan “via legitimate channels” to help Li’s family go to China to visit him and “avoid further worsening in cross-Strait relations,” the DPP said.

Li’s family and the Taiwan government have previously expressed frustration at not being told where Li was being held.

His wife, Li Ching-yu, was barred from traveling to China last month, but this month went to Washington to appeal to the U.S. Congress for President Donald Trump to press Beijing to free activists and end torture.

A group of non-governmental organizations in Taiwan, supporting Li’s wife, slammed China for its “big battle” approach against Li and said Beijing’s actions to date have seriously violated Li’s basic human rights.

Relations between China and Taiwan have cooled since Taiwan President Tsai Ing-wen took power last year, because she refuses to concede that the self-ruled island is part of China.

China views Taiwan as a wayward province and has not ruled out the use of force to bring Taiwan under its control.

Defeated Nationalist forces fled to Taiwan in 1949 after losing a civil war with the Communists.

(This story corrects paragraph 6 Taiwan’s Mainland Affairs Council, not Office)