Lisbon: Thousands of anti-abortion protesters took to the streets of Lisbon on Sunday urging Portuguese to reject a referendum aimed at lifting the ban on abortion in this deeply Catholic country.

Up to 15,000 people joined a "walk for life" that began in front of a Lisbon maternity hospital, police and organisers said. "The law has to defend the defenceless, the one that has no voice, the child that is inside his mother's womb," said Jose Bagao Felix, a conservative former government minister.

Portugal, one of a few countries in Europe that bans abortion, will hold a vote to decriminalise abortion in the first 10 weeks of pregnancy on Feb. 11. The number of Portuguese in favour of legalising abortion has fallen to 59% from 72% in recent months, according to a poll by Portugal's Catholic University.

And with the number of undecided voters rising, the result of the referendum is difficult to predict.The referendum, proposed by the ruling Socialists, needs more than 50% of the electorate to vote to be valid.

A referendum on the issue in 1998 failed when too few voters turned out. A recent poll found the number of those who will definitely vote has fallen to 58% from 68%, suggesting a repeat of 1998 is possible.