More than a million people are said to have taken part in a march in Madrid to oppose government plans to liberalise Spain's abortion law.
Several dozen centre-right opposition party joined the demonstration, which was backed by Roman Catholic bishops.
Socialist Prime Minister Jose Luis Rodriguez Zapatero wants to introduce abortion on demand.
At present, a pregnancy can only be terminated in mainly Catholic Spain under specific circumstances.
The government wants the procedure to be available to all women up until the 14th week of pregnancy.
Most controversially, the draft law currently before parliament would also permit girls aged 16 and 17 to have an abortion without their parents' knowledge.
It is the latest in a series of ethical issues which have pitted the Catholic right against the government, which has legalised gay marriage and made divorce easier.
Police estimates put the crowd at 250,000, but the regional government said that over a million had turned out, with the organisers claiming a turnout of two million.
'Every life matters'
The march brought together more than 40 religious and civil society groups calling for the government to withdraw the draft bill.
The march drew together more than 40 religious and other groups
"This new law is a barbarity," said one protester, Jose Carlos Felicidad, from the southern town of Algeciras.
"In this country, they protect animals more than human beings," he told AFP news agency.
A broad cross-section of Spanish society were represented, says the BBC's Steve Kingstone in Madrid - old and young, parents with babies, priests, nuns, immigrant families and organised groups coached in from all over the country.
They gathered in the heart of Madrid under an enormous blue banner the height of a two-storey building emblazoned with the simple message: "Every life matters."
The crowd stretched all the way up the city's main avenue in what our correspondent says was a show of strength by Spain's traditional Catholic right.
The demonstrators would have been hoping that lawmakers at the parliament nearby were listening, our correspondent adds, because it is they who in due course will vote on this controversial legislation.
Respect and rights?
Spain's existing law, dating from 1985, allows abortion in cases of rape and when there are signs of foetal abnormality.
Spanish women can also end a pregnancy if their physical or psychological health is at risk. In practice, the last category has been used to justify the vast majority of abortions - of which there were 112,000 in 2007.
The government says the new law is about respect and rights for women, and that anyone wanting to terminate a pregnancy will first be explained the alternatives - including state help for young mothers.
It also claims its proposal will make abortion safer - by ensuring the procedure does not happen beyond 22 weeks of a pregnancy.
In recent years shocking cases have emerged in which doctors performed abortions on women eight months pregnant, with the justification that their mental health was under threat.
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