The Scottish government has been urged to stand firm over its plans to legalise same-sex marriage and not be derailed by an "anti-gay agenda", as the Catholic church in Scotland launched a campaign to maintain "the universally accepted definition of marriage" as a union between a man and a woman.

In a letter read out in all 500 of the church's parishes, Scotland's Catholic bishops expressed their "deep disappointment" that Alex Salmond's administration has vowed to pass legislation that could see the first gay marriage ceremonies by 2015. The letter called on worshippers to pray for their political leaders so that they may preserve the traditional nature of marriage "for the good of Scotland and of our society".

Cardinal Keith O'Brien, the leader of the country's Catholic church, who last weekend broke off direct talks with the Scottish government on gay marriage, said: "The church's teaching on marriage is unequivocal, it is uniquely the union of a man and a woman and it is wrong that governments, politicians or parliaments should seek to alter or destroy that reality." He added: "While we pray that our elected leaders will sustain rather than subvert marriage, we promise to continue to do everything we can to convince them that redefining marriage would be wrong for society."

Gay rights activists condemned the move. "It is increasingly clear that the church has an anti-gay agenda that it wants to impose on the rest of society," said Tom French, policy co-ordinator of the Equality Network. "We urge the Scottish government to stand firm on plans to introduce equal marriage and not give in to demands that would discriminate against LGBT [lesbian, gay, bisexual and transgender] people."

Particularly worrying, said French, was any suggestion that the Catholic church would take the fight into schools. According to the bishops' letter, a National Commission for Marriage and the Family will be launched, which would be especially important for young people and children. O'Brien said the body would "develop an online presence and produce materials and organise events" to promote the cause.

French said the Equality Network would be "deeply concerned" by any attempt to take these activities into schools. "School should be a welcoming environment for all young people, regardless of their sexual orientation or their family situation," he said.

In the face of intense pressure from not only the Catholic church but also Muslim organisations and evangelical and presbyterian churches, the Scottish government has continued with its plan to legalise gay marriage. A draft bill is to be published later this year.

Opinion polls have shown that about two-thirds of Scots are in favour of the changes.

The government has insisted that no clergy would be forced to carry out same-sex weddings and has included new protections and "conscience clauses" for churches and individual clergy in the legislation. This has not, however, succeeded in placating opponents, foremost among them O'Brien, who in March drew criticism when describing gay marriage as a "grotesque subversion of a universally accepted human right".

In the letter to followers on what was declared "national marriage Sunday", the bishops said: "We write to you having already expressed our deep disappointment that the Scottish government has decided to redefine marriage and legislate for same-sex marriage. We take this opportunity to thank you for your past support in defence of marriage and hope you will continue to act against efforts to redefine it. We reaffirm before you all the common wisdom of humanity and the revealed faith of the church that marriage is a unique lifelong union of a man and a woman."

It added: "The bishops of Scotland will continue to promote and uphold the universally accepted definition of marriage as the union solely of a man and a woman."