Christian coalition petitions to stop gay marriage law
Christian campaigners are petitioning Prime Minister David Cameron to stop the introduction of same-sex marriage.
Coalition for Marriage (C4M) said it was emailing 175,000 people with a petition against the government's plan to legislate for gay marriage by 2015.
Former Archbishop of Canterbury Lord Carey backed C4M, saying the plan was a "hostile strike" on marriage.
But gay rights campaigner Peter Tatchell said C4M's stance on marriage was homophobic and discriminatory.
"The Coalition for Marriage is intolerant and out of touch," Mr Tatchell said.
"Coalition members are entitled to believe that same-sex marriages are wrong, but they are not entitled to demand that their opposition to such marriages should be imposed on the rest of society and enforced by law."
In September, Liberal Democrat Equalities Minister Lynne Featherstone announced the coalition government's proposals to legislate for same-sex marriage. David Cameron told last October's Conservative Party conference he supported gay marriage.
A public consultation on how to make civil marriage available to same-sex couples will be launched in March.
C4M's petition states: "I support the legal definition of marriage which is the voluntary union for life of one man and one woman to the exclusion of all others. I oppose any attempt to redefine it."
At a media conference in central London, Lord Carey said he had signed the petition.
"The avowed intention to widen the scope of marriage as we see before us is a hostile strike, which rather than strengthening marriage, will destroy its meaning and diminish its importance drastically," he said.
"The legal and theological definition of marriage is that of a man and a woman in a lifelong relationship. The government has many difficult duties to perform on behalf of the nation it is elected to serve. But it is not in its gift to alter such a fundamental relationship."
Lord Carey said legalising gay marriage would be "an act of cultural and theological vandalism".
In response to the launch of the petition, Ben Summerskill - chief executive of gay, lesbian and bisexual charity Stonewall - said: "Our strong advice to anyone who disagrees with same-sex marriage is not to get married to someone of the same sex."