Waving white flags, pro-life demonstrators of all ages, bussed in from across Spain, marched in the sunshine and massed near the entrance to the Retiro park, yelling "Every life matters!"
Conservative Prime Minister Mariano Rajoy promised before he was elected in 2011 to tighten Spain's abortion law, but dropped the plan last September due to disagreement within his Popular Party (PP).
The reform would have ended a woman's right to freely opt for an abortion up to 14 weeks of pregnancy, which is standard in much of Europe.
Its abandonment angered Catholic and other pro-life groups such as those that organized Saturday's rally.
"I will always do everything I can to support life, and even if some day I make a mistake, I will not have an abortion," said one demonstrator, Lourdes Keller, a 19-year-old medical student.
Rajoy's government last month proposed a new reform that would oblige girls aged 16 and 17 to get their parents' consent to have an abortion.
That failed to pacify pro-life campaigners who said it did not go far enough.
Some voters threatened to punish Rajoy in the general election due around November by abandoning the PP for parties such as the alternative right-wing movement Vox which split from the PP in 2014.
"I voted for the PP because it included defence of the right to life in its election programme," said one demonstrator, Francisco de Castro, 40, from Valencia.
"Now we have realized that they don't defend life. For that reason, they have lost my vote."
The PP seized power from the Socialists by a landslide in the 2011 election at the height of an economic crisis but Spain's electoral landscape has been transformed since then.
Alternative parties such as the left-wing Podemos and centre-right Ciudadanos have surged in opinion polls in recent months, eating away at the PP and Socialists' share of the vote.