Former judge Manuela Carmena, of the Indignados (Outraged) protester movement that grew out of Spain's economic crisis, was sworn in as mayor of Madrid on Saturday.

Carmena unseated the conservative Popular Party, which has ruled the Spanish capital for nearly a quarter of a century, after winning a majority of votes from city hall representatives.

"I promise to loyally respect the duties involved in being mayor of Madrid," Carmena said as she was sworn in, moments after the representatives' vote.

Earlier the hall broke out in applause, as supporters cried out the rallying call of the Indignados movement: "Yes, it is possible!"

Carmena became mayor after her leftist platform, Ahora Madrid, clinched a deal with the main opposition Socialists, some two weeks after after thrashing the ruling Popular Party in local and regional elections in May.

Carmona's platform includes neighbourhood associations, environmentalist groups and Spain's new anti-austerity party, Podemos, whose strong gains made them kingmakers in what analysts have described as a historic vote.