Evangelical church ministers have urged First Minister Alex Salmond not to allow same-sex couples to marry in Scotland.
They are handing over a letter at St Andrew's House in Edinburgh urging Mr Salmond to support the current definition of marriage.
Same-sex couples can enter a civil partnership which carries full legal rights but the ceremony cannot be conducted in a church or other religious premises.
A Scottish Government consultation asking if marriage should be allowed for gay people through a civil or religious ceremony closed earlier this month after attracting more than 50,000 responses.
Representatives from more than 70 of Scotland's largest evangelical churches have signed the letter which will be handed to Mr Salmond.
It states: "In supporting marriage, we are not removing rights from anyone. Redefining marriage is not an issue of equality or fairness, as argued by those calling for this change. Equality does not mean sameness but recognises diversity.
"In the Scottish Government's consultation document it is abundantly clear that homosexual couples already have the rights of marriage available through civil partnerships.
"Despite the sincere promises to protect those who disagree with redefining marriage, we have well-founded misgivings about the religious liberty implications. However, our chief concern is that Scottish society will be the poorer if the definition of marriage is rewritten. Marriage is foundational and so much else rests upon it.
"You cannot radically alter the definition of marriage without it having knock-on effects on so much else in our society."
The letter will be handed over by three signatories to the letter, Reverend Alex Macdonald, Reverend Jeremy Middleton and Reverend David McCarthy, who will then take part in a rally.