About 600 buses and several aircraft were used to bring the supporters of 42 Spanish anti-abortion and Catholic associations to the capital for the protest, which is also backed by the conservative opposition Popular party and the Roman Catholic Church.
The protesters, expected to include Jose Maria Aznar, the former prime minister, were to march about 1.5km across central Madrid.
One anti-abortion group, HazteOir (Make Yourself Heard), said demonstrations were also planned in front of Spanish embassies in other countries, including Italy, France, Poland, Ireland, the United States, Nigeria and in several Latin American nations.
"I think it is just cruel to deny [a pregnant woman] the services that give her the choices of how to deal with that situation"
The demonstration "is not just aimed at the withdrawal of the new law, the basic message is that the debate will continue as long as there is a single abortion in Spain," Benigno Blanco, the head of the Forum for the Family, one of the organisers, told the conservative newspaper ABC
The proposed abortion law, approved by the cabinet last month, would allow the procedure on demand for 16-year-olds and over up to the 14th week of pregnancy, and up to 22 weeks if there was a risk to the mother's health or if the foetus was deformed.
Women could also undergo the procedure after 22 weeks if the foetus had a serious or incurable illness.
The existing law introduced in 1985, a decade after the death of right-wing dictator Francisco Franco, allows abortion under more limited conditions.
The proposed new legislation, which is based on laws in place in most other EU countries, is to be debated in parliament in November.
Kate Smurthwaite, from the group Abortion Rights, told Al Jazeera: "People panic and flinch at the notion of a young woman having an abortion without the consent of her family.
|A series of public opinion polls has shown a majority of Spaniards oppose reform [AFP]
"But of course, you have to remember the kind of young woman who is going to be offered the love and support of her family is exactly the kind of woman who is quite willingly and freely going to talk to her parents.
"We should be asking the question, if a woman is too young to buy alcohol, then how are we allowing her to be sexually active in the first place, and how are we allowing her to run the risk of being pregnant?
"Once that woman is in pregnancy, and in crisis pregnancy, I think it is just cruel to then deny her the services that give her the choices of how to deal with that situation."
An opinion poll published in ABC on Friday said 42 per cent of Spaniards believed there was no overwhelming popular support for the reforms, compared to 38 per cent who believed there was.
A poll released earlier this month in the centrist Catalan newspaper La Vanguardia said a narrow majority of Spaniards opposed the reforms.
Jose Luis Rodriguez Zapatero, the prime minister, has defended the changes, saying the state should not "intervene in the free and private decision of a woman, who is the one who has to take on the responsibility of a pregnancy during her entire life."
Zapatero has passed a number of social laws since coming to power in 2004 that have angered the Roman Catholic Church, including measures to legalise gay marriage, allow for fast-track divorces and give increased rights to transsexuals.