An influential Protestant church in China has said two of its leaders were detained after more than dozens of congregants, including children, were rounded up for interrogation.

They were midway through Sunday service in the south-western city of Jiangyou, when armed police officers stormed the room they were in, Early Rain Covenant said in a statement on Monday.

Founded in 2008 in Chengdu city, the church has long been on the Chinese Communist Party's radar given how religion is tightly controlled in the region.

Founding pastor Wang Yi was detained in a raid in December 2018 and is serving a nine-year jail term for "inciting subversion of state power" and "illegal business operations".

The grounds for detaining two of its leaders, Yan Hong and Wu Wuqing, on Sunday are still unclear, the church said in its statement posted on Telegram. Chinese authorities have not responded to the statement, or made any comment so far.

The church also shared photographs and videos that show the congregants, seated in a hotel ballroom surrounded by a team of SWAT (Special Weapons and Tactical Unit) officers.

At least 50 police officers were present during the raid at 11:00 local time, according to some members' estimates.

More than 30 members and leaders were "forcibly taken away in several police vehicles" and questioned in the Jiangyou detention centre, the church said. Throughout the process, they "fellowshipped, sang hymns, and prayed until most of them were released," it added.

The remaining congregants, which included elderly and children, were locked up in the ballroom and subject to identity checks, according to the church. Clips show some congregants singing even as an officer in plain clothes took to the stage and repeatedly shouted for them to stop.

The church said officers tried to get those in the ballroom to sign an affidavit in exchange for their release, but did not disclose what was in the affidavit. The congregants refused and were eventually released at 18:00.

Apart from Yan and Wu, those who were taken away for interrogation were released between 21:00 and 23:00 on Sunday.

The two preachers have previously been detained by authorities, the most recent being in January, when they were summoned by police for "picking quarrels and provoking trouble".

Chinese authorities said in 2018 that there were 44 million Christians in the country, but it is unclear if this number includes those who attend the many underground churches.

The Communist Party pressures Christians to join only state-sanctioned churches led by government-approved pastors.

Many have turned to underground churches, also known as "house churches", over the years, but Christian groups say the government's grip has tightened noticeably, with arrests becoming more common.

"[Sunday's] raid is another stark reminder that the Chinese Communist Party continues to treat peaceful Christian worship as a threat to state control," says Bob Fu, founder of non-profit ChinaAid, which monitors religious persecution.

In October last year, 30 leaders of Zion Church, another one of China's biggest underground churches, were rounded up across seven cities. Its founder Ezra Jin is still in custody.