The US-based Chinese Human Rights Defenders (CHRD) said 69-year-old Gao, who is accused of slandering China's heroes, was tried behind closed doors at a court in the northern city of Sanhe, though no verdict has yet been announced.

"Gao Zhen has the right to freedom of artistic expression. The use of a contrived, retroactively applied law and a closed trial underscores serious due process violations," CHRD researcher Shane Yi said in a statement.

"The charges should be dropped and Gao Zhen released immediately."

Gao and his brother Gao Qiang gained fame in the early 2000s for art touching on sensitive political topics like the Cultural Revolution, the Tiananmen Square protests and the legacy of Mao, who led China from 1949 until his death in 1976.

Their works include a sculpture of the Communist leader kneeling, titled "Mao's Guilt", and another called "Execution of Christ", in which several bronze statues of Mao aim rifles at one of Jesus.

Gao Zhen, who emigrated to the United States in 2022, was detained at his studio on the outskirts of Beijing in August 2024 during a visit to China.

He was indicted in June last year on charges of insulting "heroes and martyrs", according to CHRD.

Since 2021, that crime has carried a maximum sentence of three years in prison, but authorities appear to be applying it retroactively to works that Gao created between 2005 and 2009.

Gao has several medical conditions, and his health has deteriorated during his detention, CHRD said.

His wife and their seven-year-old son, a US citizen, have been barred from leaving China.

The absence of a verdict on Monday indicated that "a political decision has not yet been made about the case", CHRD's co-executive director Sophie Richardson said.

"This has nothing to do with law."

The space for artistic expression in China has narrowed since President Xi Jinping came to power in 2012, with authorities often using the crime of insulting heroes and martyrs to target artists and cultural figures.