Christians are being urged to pray ahead of the third reading of the Marriage (Same Sex Couples) Bill.
The Bill is due to have its third reading on 20 and 21 May. Proposed amendments include exempting religious schools from having to promote or endorse gay marriage, the right to conscientious objection as grounds for refusing to conduct a same-sex marriage ceremony, and for a referendum to be held on the issue of same-sex marriage.
MPs supporting the amendments include David Burrowes and Fiona Bruce.
To coincide with the Bill's third reading, Christian Concern is organising prayer meetings outside Parliament at 12:00 - 14:00 and 17:00 - 19:00 on 20 and 21 May.
Speaking in the House of Lords this week, former Archbishop of Canterbury Lord Carey said the passing of the Bill had been "wholly unedifying" and that debate on its content had been "curtailed".
He was also critical of the Government for failing to listen to the views of black and ethnic minority communities.
"My Lords, had the Government listened more and not engaged in a desperate bid to paint all the opponents of this bill as elderly Christians, a strange breed of non-relevant dinosaurs, perhaps they would have started to address these issues and picked up the chorus of disapproval from those from our black and minority ethnic communities, who have been for no apparent reason, deliberately excluded from the legislative process," he said.