Well-heeled protesters, many accompanied by small children and bearing heart-shaped red balloons with the slogan "The Right to Live", marched for the second time in six months against the changes, due to become law in July.

Media estimated the crowds in the thousands. The previous march in October drew tens of thousands in one of the largest demonstrations since anti-war protests in 2003 and 2004.

The bill has galvanised opposition to Spain's minority Socialist government and its Prime Minister Jose Luis Rodriguez Zapatero, criticised for his handling of a deep economic crisis marked by high unemployment.

Demonstrators, many waving Spanish flags, marched under banners reading: "ZP (Zapatero) 007: Licence to kill" and "Spaniards in support of life".

Although decriminalised in 1985, abortion is currently only allowed under Spanish law in cases of rape, if a foetus is damaged or if the pregnancy could endanger the physical or mental health of the mother.

The new law will allow abortion in most cases up to the 14th week of gestation without risk of prosecution for women seeking termination for whatever reason.

"No woman can be sent to jail for terminating her pregnancy or threatened with that. That's the difference," said Prime Minister Zapatero at a political rally on Saturday.

The government has accelerated the transformation of conservative, Catholic Spain into a liberal society during its six years in power with legal gay marriage and a reduced role for religion.

While the number of abortions has doubled in the past decade, the practice is controversial in the traditionally Catholic country.

Maruchi Barosa, 68, accompanied by three children draped in Spanish flags, said the march was social rather than political.

"For me it's a demonstration in support of humanity," she said.