Thousands of people have taken to the streets of Madrid to protest against government moves to legalise marriage for same-sex couples.
Gay rights activists are staging their own counter-protest nearby
The protest is being led by Roman Catholic bishops and members of the conservative opposition Popular Party.
More than 500 buses were laid on to ferry people to Madrid, along with special flights from the Canary Islands and Spanish areas in Morocco.
In April the lower house voted for gay couples to marry and adopt children.
The Vatican condemned the bill, which if also passed by the Spanish Senate, will make Spain one of the few European countries to allow homosexuals to marry and to adopt children.
Belgium and the Netherlands have also legalised gay marriages.
The head of the Vatican's Pontifical Council on the Family, Cardinal Alfonso Lopez Trujillo, denounced the legislation as profoundly iniquitous.
When Socialist Prime Minister Jose Luis Rodriguez Zapatero took office over a year ago, he made it clear he intended to remove what he called the Church's undeniable advantages and make Spain a secular state.
Mr Zapatero has indicated that he also intends to streamline divorce law and even to relax the conditions placed on abortion.
Opinion polls suggest that a majority of Spaniards support the gay marriage and adoption bill.
However, the move has met resistance in the predominantly Roman Catholic nation, with several Spanish mayors saying that they will refuse to marry same sex couples if the bill becomes law.
The march has been organised by the Spanish Forum for the Family (FEF), a lay Catholic group, whose slogan for the march is "The family is important. For the right to have a mother and father. For liberty".
Gay and lesbian groups are staging their own counter protest a short distance from the march to voice their support for the bill.