The Vatican has launched a global campaign against gay marriage in an attempt to reverse the spread of legislation in Europe and the Americas that permits it.
The Vatican says homosexuality goes against "natural moral law"
In a strongly-worded 12-page document signed by the Pope's chief theological adviser, Cardinal Joseph Ratzinger, the Church brands homosexual unions as immoral, unnatural and harmful.
The Vatican's stance has been greeted with dismay by opponents who say it runs against human rights conventions and is out of touch with the modern world.
It comes a day after US President George Bush - a Methodist Christian - spoke out against the idea of same-sex marriages as church leaders met in Minneapolis to debate the appointment of a gay bishop.
"There are absolutely no grounds for considering homosexual unions to be in any way similar or even remotely analogous to God's plan for marriage and family," the Vatican
"Marriage is holy, while homosexual acts go against the natural moral law."
A small number of countries have legally recognised same-sex unions, including the Netherlands, Belgium and two provinces in Canada.
Other countries, such as France, Germany and Argentina, allow homosexual couples to register their partnerships with the local civil authorities and to obtain some of the social benefits available to heterosexual couples.
Even Catholic Croatia has recently passed a law giving homosexual couples the same legal standing as unmarried heterosexual couples.
BBC Rome correspondent David Willey says the Pope is worried that other European Union countries, including Italy, will follow suit and legalise gay marriages.
However supporters of gay rights were quick to respond.
Near the Vatican's St Peter's Square, a small group of demonstrators from Italy's Radical Party held up banners reading "No Vatican, No Taleban".
Another critic, Italian parliamentarian and gay rights activist Franco Grillini, said the document was part of a "homophobic crusade" by the Vatican.
Other opposition came from Germany's Green Party - a junior partner in the government of Chancellor Gerhard Schroeder.
Senior Green official Volker Beck on Thursday condemned "a sad document of closed-mind fanaticism".
'Sanctity of marriage'
The issue of same-sex marriages is particularly charged in the United States, where some lawmakers in the House of Representatives have proposed a constitutional ban on gay marriages to counter state laws granting legal recognition to gay unions.
On Wednesday President Bush said: "I believe in the sanctity of marriage. I believe a marriage is between a man and a woman, and I think we ought to codify that one way or the other."
Cardinal Ratzinger wants to bolster heterosexual marriage
His remarks were seen as offering a sop to conservatives who were angered earlier this month after he distanced himself from the House proposal for a constitutional ban on gay marriage.
The Vatican document, entitled Considerations Regarding Proposals to Give Legal Recognition to Unions Between Homosexual Persons, sets out a battle plan for politicians opposed to legislation permitting gay marriage and adoption by gay people.
Catholic politicians have a "moral duty" to publicly oppose such legislation and to vote against it in parliament, it says.
"To vote in favour of a law so harmful to the common good is gravely immoral."
Non-Catholics are also urged to join the campaign to "defend the common good of society".
Our correspondent says the new Vatican document, published by the Vatican's orthodoxy watchdog, the Congregation for the Doctrine of the Faith, repeats arguments already given by the Catholic Church against same-sex unions.
It was published on the same day that the Israeli city of Tel Aviv granted same-sex couples the same discounts as married couples in what gay activists hailed as a step towards full integration in the Jewish state.
Gay residents of Tel Aviv who declare their union in a statement will be authorised to receive discounts for city services and sites such as sports centres and museums.