Authorities in China have detained the pastor of the country’s largest official Protestant church, who publicly objected to a government drive to demolish crosses on church buildings, in what rights groups say is part of a larger crackdown on religious freedom in the communist nation.

Pastor Gu Yuese, also known as Joseph Gu, was placed under “residential surveillance in a designated location” — the official term for facilities known more commonly as “black jails” — in the city of Hangzhou last Thursday, according to U.S.-based Christian rights group China Aid.

Gu, who headed Hangzhou’s prominent Chongyi Church, was reportedly removed from his post by China’s Three-Self Patriotic Movement (TSPM), the Chinese Communist Party–sanctioned authority that regulates churches, 10 days prior to his detention. He had been vocal in his opposition to the government’s destruction of crosses atop church buildings in China’s Christian-dominated eastern province of Zhejiang, a campaign that began in 2014.

“His arrest marks a major escalation in the crackdown against those who oppose the forced demolition of crosses,” Bob Fu, president and founder of China Aid, said in a statement. “He will be the highest-ranking national church leader arrested since the Cultural Revolution.”

Read more: Expansion of Christian Church in the Birthplace of Confucius Creates Controversy in China

A TSPM official, however, refuted reports of Gu’s detention when contacted by Radio Free Asia, claiming that the Christian leader was simply “helping with an investigation.”

The pastor’s wife Zhou Lianmei has also gone missing, and family members believe she has been taken into custody as well.

Gu’s imprisonment has reportedly caused a wave of alarm across China’s evangelical community, with growing fears that the authoritarian government’s increasing proclivity to silence dissenting voices has extended to the religious sphere.

“This is really quite an escalation,” Fu told the Christian Science Monitor. “It sends a signal to silence any potential future dissenting voices from within the church. It tells everyone to shut up.”

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Write to Rishi Iyengar at rishi.iyengar@timeasia.com.