Police have detained dozens of churchgoers and leaders of one of China's most prominent Protestant house churches, congregation members and activists said, in the latest government action against unregistered religious groups.

Teams of police and state security officials in the southwestern city of Chengdu came to the homes of leaders and members of the Early Rain Covenant Church late on Sunday and detained them, according to church announcements sent to members and passed to Reuters by churchgoers and activists.

'The time has come. O, Lord! Did you not establish this church for this very purpose?' the church said in one of its announcements.

'We will wait for Thee, as one keeping vigil waits for daybreak.'

More than 100 people are believed to have been detained, said a church elder, who declined to be identified.

Neither the Chengdu nor national-level Public Security Ministries responded to faxed requests for comment.

China's constitution guarantees religious freedom, but since President Xi Jinping took office six years ago, the government has tightened restrictions on religions seen as a challenge to the authority of the ruling Communist Party.

Chinese law requires that all places of worship register and submit to government oversight, but some churches have declined to register, for various reasons.

The Early Rain Church is one of the best known unregistered house churches in China.

Members of the church have been unable to contact pastor Wang Yi, the church's founder, or his wife, and church groups on the instant messaging platform WeChat had also been blocked, the church said in an announcement.

Reuters was unable to reach Wang for comment.

Beijing-based journalist Ian Johnson, who wrote about Wang and his congregation in his 2017 book The Souls of China, said on Twitter that the crackdown was 'really disturbing news'.

'I'm afraid this is part of a bigger crackdown on unregistered Christian churches as the government pushes its efforts to dominate society,' he wrote.

Police told Zhang Xianchi, 84, a well-known author and church member, not to visit the church, because it had been 'outlawed' and that all its top leaders had been detained, according to an account from Zhang shared with Reuters by church members.

A video posted online by activists in China and overseas, which Reuters was unable to independently verify, showed a group of about a dozen plainclothes policemen taking church-goers away from a meeting.

In the footage, police told a woman they were taking a man to the municipal public security bureau to be investigated, while a child cried in the background.

'I'm telling you, we are enforcing the law,' a policeman said to the woman.

Churches across China have more pressure to register since a new set of regulations to govern religious affairs came into effect in February and increased punishments for unofficial churches.