Waving white flags with the slogan "Every life counts", demonstrators of all ages from across the country marched through the capital, whistling angrily as they passed the headquarters of the ruling Popular Party.
"Killers! Abortionists!" some yelled. "Yes to life, no to abortion!"
Prime Minister Mariano Rajoy had promised before he was elected in 2011 to tighten Spain's abortion law, but he dropped the plan last September due to disagreement within his own party.
The architect of the reform, Alberto Ruiz-Gallardón, resigned as Rajoy's justice minister.
The retreat angered many of the party's supporters, who threatened to abandon him in the general election due a year from now.
"Rajoy, we may not vote," read a huge white banner at Saturday's march.
"I am disappointed. This may change the vote," said José Vicente Romero, 50, a PP activist who travelled several hours from Alicante with a group from his church.
"Abortion is not a right. Abortion is a tragedy," said Romero, a red and yellow Spanish flag resting on his shoulder.
Ruiz-Gallardón's reform would have ended a woman's right to freely opt for an abortion up to 14 weeks of pregnancy, standard in much of Europe.
He proposed to allow abortion only in cases of rape reported to the police, or a medically certified threat to the mother's physical or psychological health.
The controversy prompted angry street demonstrations by supporters and opponents of the reform.
Among Saturday's demonstrators was Amparo Bonastre, a 17-year-old schoolgirl who came by bus from the eastern city of Valencia.
"It does not affect me personally because I would never have an abortion. But if there were a reform, lots of people would be allowed to live, without having their lives cut short," she said. "Life must be defended."
The anti-abortion groups that organised the rally demanded that the government resurrect the reform and improve adoption procedures.
"The government and its party are betraying their electoral commitments and their voters," they wrote in a manifesto.
Dropping the reform "would confirm the total lack of protection for the two victims of abortion: the unborn child, who lacks judicial protection, and the mother, who is given no alternative".