Tens of thousands of demonstrators crowded central Madrid on Sunday, waving banners and chanting slogans against government plans to liberalise the country's abortion laws.
Protestors massed outside the Equality Ministry, which is drafting a law, and marched through the streets with signs proclaiming: "There is no right to kill, there is the right to live" and "Women yes, abortion no".
"Get out of here and let the children live," they chanted, calling on the socialist government's equality minister Bibiana Aido to resign.
A spokesman for groups organising the protest, which included Right to Life and Doctors For Life, said there were 500,000 demonstrators. AFP reporters estimated the number at up to 100,000. Police did not immediately give a figure.
Elderly people marched alongside young families with toddlers in pushchairs as the protest wound its way through the city centre.
Many wore red baseball caps distributed by pro-life groups as a symbol "of strength and the celebration of life."
"As a Catholic, I think we should help women have children, not abort them," said one protestors, Paco Ortega, 42.
In one part of the demonstration, children in red caps danced and sang: "Thank you mummy for letting me live."
The protest was the first in a series of demonstrations against the abortion reform by the socialist government of Prime Minister Jose Luis Rodriguez Zapatero.
It wants to offer greater legal protection to women who wish to have an abortion and doctors who carry out the procedure.
Spain decriminalised abortion in 1985 but only for certain cases: up to 12 weeks of pregnancy after a rape; up to 22 weeks in the case of malformation of the foetus; and at any point if the pregnancy represented a threat to the physical or mental health of the woman.
The government also proposes to give girls from the age of 16 the right to have an abortion without their parents' consent.
Earlier this month a government-appointed panel of experts recommended that Spain ease its restrictions on abortion and allow the procedure on demand up to 14 weeks into a pregnancy.
"The government wants to approve a free abortion law that leaves the unborn completely unprotected," Right to Life spokeswoman Gador Joya told the crowd at the end of the march.
The proposed law "will only lead to more deaths and more suffering by thousands of women," she said. "We demand that our laws protect the right to live and to be a mother."
The march was backed by the right-wing opposition and by Spain's Roman Catholic Church, which has clashed with the government before over the legalisation of gay marriage. There were also mass demonstrations against a 2005 law which allowed homosexuals and lesbians to marry.
The church launched this month a new anti-abortion campaign that says threatened species of animals are better protected than the unborn.
The controversial campaign is illustrated by a poster showing a baby beside an Iberian lynx cub, a protected species in Spain, with the words "Protect the lynx." A caption above the baby says, "And me?"
Only two percent of abortions in Spain are estimated to take place in public clinics, where many doctors refuse to perform them on ethical grounds or because they fear legal action.
A Spanish feminist coalition on Friday launched a campaign to support the government plans.
"We are going to flood Spain with posters in response to the church's alarmist campaign," said Angeles Alvarez, spokeswoman for the State Network of Feminist Organisations, which groups over 200 associations.