A Tibetan language activist who appeared in a New York Times video has been put on trial for separatism in proceedings dismissed as a “sham” by rights groups, a sign of increasingly hardline attitudes towards government critics.

Tashi Wangchuk pleaded not guilty to the charges of “inciting separatism” during the four-hour trial in the western Chinese city of Yushu, where the state’s main piece of evidence against him was the nine-minute video, according to his lawyer, Liang Xiaojun.

The activist was detained two years ago after appearing in the video where he criticised policies towards Tibetan language education, attempted to sue the local government and tried to entice China’s state broadcaster, CCTV, to report on his concerns.

He faces up to 15 years in prison if convicted, and the judge said he would deliver a verdict at a later date. China’s Communist party-controlled courts have a conviction rate of more than 99%.

Tashi had advocated for greater use of Tibetan in education, bemoaning the use of Mandarin for most instruction in classrooms where the majority of students are Tibetan. Many Tibetans view language policies as an attempt by the government to destroy their culture.

In the New York Times video, he travels to Beijing and tries to file a lawsuit against officials in Yushu, but is ultimately turned away. “I want to try to use the People’s Republic of China’s laws to solve the problem,” he said in the video.

Rights groups described the case against him as “ludicrously unjust”.

Roseann Rife, East Asia research director at Amnesty International, said: “It is appalling that Tashi Wangchuk could face up to 15 years’ imprisonment simply for expressing his views in media interviews.” Exposing and criticising the suppression by government of Tibetan language and culture was a legitimate exercise of free speech, she added. “Labelling it as a form of ‘inciting separatism’ demonstrates how the Chinese authorities blatantly misuse this criminal charge to silence dissent.”

The Chinese government has instituted harsh policies in its far western regions, home to millions of ethnic minority citizens, in an effort to quell dissent by force. Even peaceful calls for greater autonomy from Beijing – technically guaranteed under Chinese law – are often met with punishment.

Sophie Richardson, China director at Human Rights Watch, said: “All Tashi Wangchuk has done is peacefully advocate for constitutionally-guaranteed rights. If Chinese authorities consider that inciting separatism, it’s hard to tell what isn’t.”