The Convener of the National Day of Prayer and Worship is asking Christians to pray and fast as the House of Lords debates legalising gay marriage.
Peers are expected to vote on the controversial legislation on Tuesday between 5pm and 7pm.
Dr Jonathan Oloyede said: "I urge you to join us as we fast and pray that God's pattern for marriage will be defended... May the Lord have mercy on his church and the United Kingdom."
The second reading of the bill has been taking place over today and yesterday. Addressing the House of Lords on Monday, the Archbishop of Canterbury the Most Reverend Justin Welby said he could not support the legislation.
"It confuses marriage and weddings. It assumes that the rightful desire for equality – to which I've referred supportively – must mean uniformity, failing to understand that two things may be equal but different. And as a result it does not do what it sets out to do," he said.
Colin Hart, campaign director of the Coalition for Marriage, accused the Government of seeking to "curtain scrutiny and crush opposition".
"While it would be unusual for the House of Lords to vote down a Government bill at second reading, the democratic deficit, the appalling truncated process, the fact that it is a free vote and the veiled threats to peers to vote for the bill or else, I hope will rebound on the Government," he said.