Christians are praying outside Parliament on Monday and Tuesday as the Marriage (Same Sex Couples) Bill has its Third Reading.
The prayer meetings have been organised by campaign group Christian Concern, which opposes any change to the law on marriage.
The gatherings are taking place on both days from 12pm to 2pm and 5pm to 7pm in Old Palace Yard, opposite the Houses of Parliament.
The call to prayer is being supported by Jonathan Oloyede, Convener of the National Day of Prayer, which brought 30,000 Christians to Wembley Stadium for a day of prayer for the nation last year.
MPs are being asked to consider an amendment to the bill providing for a referendum to be held on gay marriage on the same day as the 2015 general election.
The amendment has been tabled by David Burrowes, Tim Loughton and Jim Shannon.
Chief executive of Christian Concern Andrea Minichiello Williams said: "It's widely known that this bill has been introduced without any mandate.
"It wasn't in any of the major parties's manifestos, the Coalition Agreement or the Queen's Speech. The whole process has been thoroughly undemocratic.
"People have never been given the chance to vote on this even though it has big social and constitutional consequences. MPs should therefore back this referendum."
Orthodox group Anglican Mainstream said in a statement today that the bill "sends out the message that fathers are dispensable for children".
"It completely decouples marriage from procreation, consummation and adultery. Once marriage is redefined and reduced to only an intimate relationship, on what logical basis could the law turn away three people who say they want to get married?" read the statement by convenor Dr Philip Giddings and secretary Canon Dr Chris Sugden.
"Government has a strong interest in supporting man-woman marriage because that has been shown to be the best context for raising children who grow up to be good citizens for the next generation."
They go on to say that churches "should not be fooled" by promises of legal protection from having to conduct same-sex marriage ceremonies.
"This will certainly be challenged in the name of 'equality'. Neither should they be fooled by the suggestion that any future government would safeguard such 'protections'
"The great danger is that people who believe in traditional marriage will be punished for their views in their careers and in accessing public services such as fostering or adoption. Government has no business in redefining marriage," they said.