Supporters will stage a wedding outside the Scottish Parliament before handing in a petition to MSPs and letter to First Minister Alex Salmond.
The rally comes the day after both sides of the controversial debate clashed on the future of the way society defines marriage.
Cardinal Keith O’Brien, the most senior Catholic in Scotland, said bishops will drum up funds on a so-called Support Marriage Sunday earmarked for 26 August.
He said: “We will use this opportunity to remind Catholics of the importance of marriage as a union of a man and a woman and to urge them to be generous in contributing to a special collection which will be used to support initiatives in defence of marriage.”
Tom French, policy co-ordinator of the Equality Network, said: “For the Catholic Church to spend another £100,000 fighting LGBT (lesbian, gay, bisexual and transgender) equality in its self-declared war on gay marriage seems to be an aggressive, undemocratic and un-Christian thing to do.
“At a time of financial crisis, when poverty and depravation are on the rise, surely there are better things that this money could be spent on.”
He said most Scots support same-sex marriage.
The Scottish Government is expected to publish the results of a recent public consultation on the issue this month.