Beijing police have arrested dozens of Christian worshippers from a "house church" - one not formally recognised by the government - when they tried to pray outdoors, a rights group said.

They sang hymns and said prayers as police loaded them onto waiting buses in Beijing's western Haidian district, the US-based Christian rights group China Aid said in a statement, citing witnesses.

"The Beijing authorities have again demonstrated their total disregard of their citizens' constitutionally guaranteed fundamental right to religious freedom," China Aid founder and president Bob Fu said in the statement.

Police declined to comment to agency reporters and requested written questions be sent to them by fax.

The New York Times reported that one if its photographers was among those detained, but was later released.

China Aid said more than 100 were detained, but the newspaper said "dozens" of people were held.

The United States and the United Nations have expressed serious concerns in the past week at a growing crackdown across China in which artists, lawyers, writers, activists and intellectuals have been detained.

The church incident comes a week after Ai Weiwei, an outspoken artist who helped design the Bird's Nest Olympic Stadium for the 2008 Beijing Games, was detained for unspecified "economic crimes."

The church tried to hold services in the open air after it was evicted from a rented space as the landlord was pressured to not renew the lease, China Aid said.

Shouwang, one of Beijing's largest house churches, invited its members to meet Sunday morning at an open air public platform linking the SinoSteel Building and the South China Poetic Restaurant building, China Aid said.

Some of the detainees were taken to a nearby elementary school where authorities took down their names and other details, the statement said.

Several church leaders had been called to their local police stations on Saturday, with some spending the night in detention while other were told they were not permitted to leave their homes on Sunday, the statement said.

China only allows religious worship in state-approved churches.

Organisers of underground churches are routinely sent to labour camps without trial.

- AFP