Release International is inviting churches to hold an open-air service across the United Kingdom next month in solidarity with Christians around the world who are forced to meet outside because of state interference.
Christians in parts of China, Indonesia and central Asia have been barred from meeting indoors because their church buildings were closed down by the authorities.
Shouwang Church, one of Beijing's largest house churches, has been meeting outside for a year after the authorities refused to register the church. Churchgoers are routinely arrested as they hold services outside.
A church spokesman said: "Even though we've been through months of battles and are feeling tired in body and soul, our desire to worship God is unchanged."
In Indonesia, the Filadelfia Batak Church, near Jakarta, has been worshipping outside since mass protests by Islamist hardliners led to its closure. The Yasmin Protestant church of West Java has also had to worship in the open air after the mayor opposed construction plans.
Churches in Britain are being asked to take part in the "Great Outdoors Church Service" on May 20 or 27.
"Would you be willing to worship in the open air in all weathers if the authorities closed down your church?" said Release chief executive Andy Dipper.
"And would you be prepared to risk arrest – every single week – just for gathering to praise God together?
"To show our support for these persecuted Christians we are inviting churches throughout the UK and Ireland to stand in solidarity with their brothers and sisters by holding a Sunday meeting outdoors."
Although it may be too difficult for some churches to meet in the open air, Dipper encouraged Christians to join in praying for those denied the freedom to worship indoors.
"Please pray that these courageous Christians would continue to be strong in the Lord; that they would know divine wisdom in how to relate to the authorities and respond with grace to those who oppose them.
"Pray that they would know God's presence as they meet outdoors and that their witness would speak strongly to the authorities of God's great love."