Ex-judge Manuela Carmena was quietly enjoying her retirement when friends called asking her to run for mayor of Madrid. She told them she was too old.

Just a few months later, the 71-year-old grandmother was standing before a crowd of supporters who were yelling "Manuela for mayor!"

She had finished second in the city election at the head of Ahora Madrid, a group spawned by the "Outraged" protest movement that made front pages worldwide in 2011.

On Saturday she will be sworn in as mayor after Ahora Madrid struck a deal with the Socialists who came in third place in local elections held on May 24.

She is one of the first to win public office as a candidate for an "Outraged" group, backed by Podemos, the political heir to the street protest movement.

Carmena claims a career spent challenging dictatorship and human rights abuses before she stepped up to fight for change in Madrid city hall.

Soft-spoken but with a piercing gaze, she stood up to her more seasoned rival, conservative Popular Party candidate Esperanza Aguirre.

Carmena did not shy from criticising even Podemos, which she said backed her as a candidate for her age and experience to take on Aguirre.

"We elderly people do not have a future, but we do have a past and I want to dedicate it to you, the young people," she was quoted as saying in a recent interview with online newspaper El Confidencial.

Carmena's mild manner belies her serious and sometimes dangerous past as a senior judge.

She gets around on a bicycle these days but once had to go everywhere with a bodyguard.

A far-right group launched an attack on her firm in 1977 which killed several of her colleagues.

When Aguirre accused Carmena of freeing from prison a member of the armed Basque separatist group ETA, Carmena retorted that she herself had also been named as a target by the group.

- Fought against Franco -

Ahora Madrid has the formal backing of Podemos, which has surged in the polls to become a contender in this year's general election.

But Carmena has insisted her group is not a branch of that party.

"Ahora Madrid is not identified with any party," she told El Confidencial.

She described Ahora Madrid as a "political platform", a non-hierarchical grouping of members from various parties, local associations and protest movements such as the Indignados.

Among other campaign pledges, Carmena has promised to stamp out corruption, develop public transport, increase subsidies for poor families and slash the mayor's salary by more than half to 45,000 euros ($50,000).

She was branded a "red" for joining the outlawed Communist party under dictator General Francisco Franco in the 1960s.

She joined, she said, "to fight against Franco". The move got her thrown out of university.

She ended up graduating nevertheless in 1965 and founded a law firm dedicated to defending the labour movement.

She held several senior posts in the judiciary and in the late 2000s chaired a working group of the United Nations Human Rights Council on arbitrary detentions, before retiring from the judiciary in 2010.

She was several years into retirement when activists last year encouraged her to run for mayor.

At first she refused, thinking herself too old, she told the La Sexta television channel.

"But the calls started getting more frequent. Friends started telling me: 'Get involved, girl. They need someone with experience with lots of ideas.'"

Born into a family of Madrid shopkeepers, she said, from an early age, "I knew I had to make the world a better place."