A bishop and a priest from China's underground Catholic church have been arrested and dozens of other Catholics were detained after they protested the police action, a US-based rights group said Thursday.

Yao Liang, 82, the auxiliary bishop of Xiwanzi diocese in the northern province of Hebei, was arrested last Sunday for the second time in 18 months, according to the Cardinal Kung Foundation.

The group said it was unaware of the reason for Yao's arrest but he had also been taken into police custody in March last year.

Li Huisheng, a 33-year-old priest from Yao's diocese, was arrested Tuesday and his whereabouts were still unknown, the group said.

The local underground Catholic community protested their arrests and 500 police were mobilized early Wednesday, detaining about 90 protesters, it said.

Around 70 of them have since been released while the rest were still under police detention, the group said, adding that two men were seriously injured and a pregnant woman had a miscarriage during the police action.

A staff member surnamed Tan at Zhangbei county's public security bureau declined to comment Thursday, saying it was a confidential matter.

Hebei province's ethnic and religious affairs bureau also refused to confirm the report.

"I have not heard of this, but if they are illegal the public security bureau would intervene," a staff member surnamed Yang told AFP.

Catholic Christians are officially only allowed to worship in the state-sanctioned churches. Beijing insists on having a say in appointing bishops — a condition that is unacceptable to the Holy See and many Chinese worshippers.

Many of the leaders of these churches have been arrested and jailed, while the churches have also been regularly raided and shut down.

Relations between China and the Vatican have been even tenser than usual in recent months after Beijing appointed a number of bishops without the Holy See's approval.